MR. JUSTICE STEWART delivered the opinion of the Court.In this case, we are called upon to resolve a dispute between the State of Washington and the Yakima Indian Nation over the validity of the State's exercise of jurisdiction on the Yakima Reservation.  In 1963, the Washington Legislature obligated the State to assume civil and criminal jurisdiction over Indians and Indian territory within the State, subject only to the condition that, in all but eight subject matter areas, jurisdiction would not extend to Indians on trust or restricted lands without the request of the Indian tribe affected.  Ch. 36, 1963 Wash.Laws. [Footnote 1]  The Yakima Nation Page 439 U. S. 466 did not make such a request.  State authority over Indians within the Yakima Reservation was thus made by Chapter 36 to depend on the title status of the property on which the offense or transaction occurred and upon the nature of the subject matter.The Yakima Nation brought this action in a Federal District Court challenging the statutory and constitutional validity of the State's partial assertion of jurisdiction on its Reservation.  The Tribe contended that the federal statute upon which the State based its authority to assume jurisdiction over the Reservation, Pub.L. 280, [Footnote 2] imposed certain procedural requirements, with which the State had not complied -- most notably, a requirement that Washington first amend its own constitution -- and that, in any event, Pub.L. 280 did not Page 439 U. S. 467 authorize the State to assert only partial jurisdiction within an Indian reservation.  Finally, the Tribe contended that Chapter 36, even if authorized by Congress, violated the equal protection and due process guarantees of the Fourteenth Amendment.The District Court rejected both the statutory and constitutional claims and entered judgment for the State. [Footnote 3]  On appeal, the contention that Washington's assumption of only partial jurisdiction was not authorized by Congress was rejected by the Court of Appeals for the Ninth Circuit, sitting en banc.  The en banc court then referred the case to the original panel for consideration of the remaining issues.  Confederated Bands and Tribes of the Yakima Indian Nation v. Washington, 550 F.2d 443 (Yakima I). [Footnote 4]  The three-judge Page 439 U. S. 468 panel, confining itself to consideration of the constitutional validity of Chapter 36, concluded that the "checkerboard" jurisdictional system it produced was without any rational foundation, and therefore violative of the Equal Protection Clause of the Fourteenth Amendment.  Finding no basis upon which to sever the offending portion of the legislation, the appellate court declared Chapter 36 unconstitutional in its entirety, and reversed the judgment of the District Court.  Confederated Bands and Tribes of the Yakima Indian Nation v. Washington, 552 F.2d 1332 (Yakima II).The State then brought an appeal to this Court.  In noting probable jurisdiction of the appeal, we requested the parties to address the issue whether the partial geographic and subject matter jurisdiction ordained by Chapter 36 is authorized by federal law, as well as the Equal Protection Clause issue.  435 U.S. 903. [Footnote 5] Page 439 U. S. 469IThe Confederated Bands and Tribes of the Yakima Indian Nation comprise 14 originally distinct Indian tribes that joined together in the middle of the 19th century for purposes of their relationships with the United States.  A treaty was signed with the United States in 1855, under which it was agreed that the various tribes would be considered "one nation," and that specified lands located in the Territory of Washington would be set aside for their exclusive use.  The treaty was ratified by Congress in 1859.  12 Stat. 951.  Since that time, the Yakima Nation has, without interruption, maintained its tribal identity.The Yakima Reservation is located in the southeastern part of the State of Washington, and now consists of approximately 1,387,505 acres of land, of which some 80% is held in trust by the United States for the Yakima Nation or individual members of the Tribe.  The remaining parcels of land are held in fee by Indian and non-Indian owners.  Much of the trust acreage on the Reservation is forest.  The Tribe receives the bulk of its income from timber, and over half of the Reservation is closed to permanent settlement in order to protect the forest area.  The remaining lands are primarily agricultural.  Page 439 U. S. 470 There are three incorporated towns on the Reservation, the largest being Toppenish, with a population of under 6,000.The land held in fee is scattered throughout the Reservation, but most of it is concentrated in the northeastern portion, close to the Yakima River and within the three towns of Toppenish, Wapato, and Harrah.  Of the 25,000 permanent residents of the Reservation, 3,074 are members of the Yakima Nation, and tribal members live in all of the inhabited areas of the Reservation. [Footnote 6]  In the three towns -- where over half of the non-Indian population resides -- members of the Tribe are substantially outnumbered by non-Indian residents occupying fee land.Before the enactment of the state law here in issue, the Yakima Nation was subject to the general jurisdictional principles that apply in Indian country in the absence of federal legislation to the contrary.  Under those principles, which received their first and fullest expression in Worcester v. Georgia, 6 Pet. 515,  31 U. S. 517, state law reaches within the exterior boundaries of an Indian reservation only if it would not infringe "on the right of reservation Indians to make their own laws and be ruled by them."  Williams v. Lee, 358 U. S. 217,  358 U. S. 219-220. [Footnote 7]  As a practical matter, this has meant that criminal offenses by or against Indians have been subject only to federal or tribal laws, Moe v. Salish & Kootenai Tribes, 425 U. S. 463, except where Congress, in the exercise of its plenary and exclusive power over Indian affairs, has "expressly Page 439 U. S. 471 provided that State laws shall apply."  McClanahan v. Arizona State Tax Comm'n, 411 U. S. 164,  411 U. S. 170-171.Public Law 280, upon which the State of Washington relied for its authority to assert jurisdiction over the Yakima Reservation under Chapter 36, was enacted by Congress in 1953 in part to deal with the "problem of lawlessness on certain Indian reservations, and the absence of adequate tribal institutions for law enforcement."  Bryan v. Itasca County, 426 U. S. 373,  426 U. S. 379; H.R.Rep. No. 848, 83d Cong., 1st Sess., 5-6 (193).  The basic terms of Pub.L. 280, which was the first federal jurisdictional statute of general applicability to Indian reservation lands, [Footnote 8] are well known. [Footnote 9]  To five States, it effected Page 439 U. S. 472 an immediate cession of criminal and civil jurisdiction over Indian country, with an express exception for the reservations of three tribes.  Pub.L. 280, §§ 2 and 4. [Footnote 10]  To the remaining Page 439 U. S. 473 States it gave an option to assume jurisdiction over criminal offenses and civil causes of action in Indian country without consulting with or securing the consent of the tribes that Page 439 U. S. 474 would be affected.  States whose constitutions or statutes contained organic law disclaimers of jurisdiction over Indian country were dealt with in § 6. [Footnote 11]  The people of those States were given permission to amend "where necessary" their state constitutions or existing statutes to remove any legal impediment to the assumption of jurisdiction under the Act.  All others were covered in § 7. [Footnote 12]The Washington Constitution contains a disclaimer of authority over Indian country, [Footnote 13] and the State is, therefore, one of those covered by § 6 of Pub.L. 280.  The State did not take any action under the purported authority of Pub.L. 280 until 1957.  In that year, its legislature enacted a statute which obligated the State to assume criminal and civil jurisdiction over any Indian reservation within the State at the request of the tribe affected. [Footnote 14]  Under this legislation, state jurisdiction was requested by and extended to several Indian tribes within the State. [Footnote 15] Page 439 U. S. 475In one of the first prosecutions brought under the 1957 jurisdictional scheme, an Indian defendant whose tribe had consented to the extension of jurisdiction challenged its validity on the ground that the disclaimer clause in the state constitution had not been amended in the manner allegedly required by § 6 of Pub.L. 20.  State v. Paul, 53 Wash. 2d 789, 337 P.2d 33.  The Washington Supreme Court rejected the argument, construing the state constitutional provision to mean that the barrier posed by the disclaimer could be lifted by the state legislature. [Footnote 16]In 1963, Washington enacted Chapter 36, the law at issue in this litigation. [Footnote 17]  The most significant feature of the new statute was its provision for the extension of at least some jurisdiction over all Indian lands within the State, whether or not the affected tribe gave its consent.  Full criminal and civil jurisdiction to the extent permitted by Pub.L. 280 was extended to all fee lands in every Indian reservation and to trust and allotted lands therein when non-Indians were involved.  Except for eight categories of law, however, state jurisdiction was not extended to Indians on allotted and trust lands unless the affected tribe so requested.  The eight jurisdictional categories of state law that were thus extended to all parts of every Indian reservation were in the areas of compulsory school attendance, public assistance, domestic relations, Page 439 U. S. 476 mental illness, juvenile delinquency, adoption proceedings, dependent children, and motor vehicles. [Footnote 18]The Yakima Indian Nation did not request the full measure of jurisdiction made possible by Chapter 36, and the Yakima Reservation thus became subject to the system of jurisdiction outlined at the outset of this opinion. [Footnote 19]  This litigation followed.IIThe Yakima Nation relies on three separate and independent grounds in asserting that Chapter 36 is invalid.  First, it argues that, under the terms of Pub.L. 280, Washington was not authorized to enact Chapter 36 until the state constitution had been amended by "the people" so as to eliminate its Art. XXVI, which disclaimed state authority over Indian lands. [Footnote 20]  Page 439 U. S. 477 Second, it contends that Pub.L. 280 does not authorize a State to extend only partial jurisdiction over an Indian reservation.  Finally, it asserts that Chapter 36, even if authorized Page 439 U. S. 478 by Pub.L. 280, violates the Fourteenth Amendment of the Constitution.  We turn now to consideration of each of these arguments.IIIWe first address the contention that Washington was required to amend its constitution before it could validly legislate under the authority of Pub.L. 280.  If the Tribe is correct, we need not consider the statutory and constitutional questions raised by the system of partial jurisdiction established in Chapter 36.  The Tribe, supported by the United States as amicus curiae, [Footnote 21] argues that a requirement for popular amendatory action is to be found in the express terms of § 6 of Pub.L. 280 or, if not there, in the terms of the Enabling Act that admitted Washington to the Union. [Footnote 22]  The Page 439 U. S. 479 argument can best be understood in the context of the specific statutory provisions involved.AThe Enabling Act under which Washington, along with the States of Montana, North Dakota, and South Dakota, gained entry into the Union, was passed in 1889. [Footnote 23]  Section 4 of that Page 439 U. S. 480 Act required the constitutional conventions of the prospective new States to enact provisions by which the people disclaimed title to lands owned by Indians or Indian tribes and acknowledged that those lands were to remain "under the absolute jurisdiction and control of" Congress until the Indian or United States title had been extinguished.  The disclaimers were to be made "by ordinances irrevocable without the consent of the United States and the people of said States."  Washington's constitutional convention enacted the disclaimer of authority over Indian lands as part of Art. XXVI of the state constitution. [Footnote 24]  That Article, captioned "Compact with Page 439 U. S. 481 the Court the United States," is prefaced with the statement -- precisely tracking the language of the admitting statute -- that "the following ordinance shall be irrevocable without the consent of the United States and the people of [the Sate of Washington]."  Its substantive terms mirror the language used in the enabling legislation.  We have already noted that two distinct provisions of Pub.L. 280 are potentially applicable to States not granted an immediate cession of jurisdiction.  The first, § 6, without question applies to Washington and the seven other States admitted into the Union under enabling legislation requiring organic law disclaimers similar to that just described.  This much is clear from the legislative history of Pub.L. 280, [Footnote 25] as well as from the express language of § 6.  That section provides,"Notwithstanding the provisions of any Enabling Act for the admission of a State, the consent of the United States is hereby given to the people of any State to amend, where necessary, their State constitution or existing statutes, as the case may be, to remove any legal impediment to the assumption of civil and criminal jurisdiction in accordance with the provisions of this Act:  Provided, That the provisions of this Act shall not become effective with respect to such assumption of jurisdiction by any such State until the people thereof have appropriately amended their State constitution or statutes as the case may be."All other States were covered by § 7.  In that section, Congress gave the consent of the United States"to any other State . . . to assume jurisdiction at such Page 439 U. S. 482 time and in such manner as the people of the State shall, by affirmative legislative action, obligate and bind the State to assumption thereof."These provisions appear to establish different modes of procedure by which an option State, depending on which section applies to it, is to accept the Pub.L. 280 jurisdictional offer.  The procedure specified in § 7 is straightforward:  affirmative legislative action by which the State obligates and binds itself to assume jurisdiction.  Section 6, in contrast, is delphic.  The only procedure mentioned is action by the people "to amend . . . their State constitutions or existing statutes, as the case may be" to remove any legal impediments to the assumption of jurisdiction.  The phrase "where necessary" in the main clause suggests that a requirement for popular -- as opposed to legislative -- action must be found if at all in some source of law independent of Pub.L. 280.  The proviso, however, has a different import.BThe proper construction to be given to the single inartful sentence in § 6 has provoked chapters of argument from the parties.  The Tribe and the United States urge that, notwithstanding the phrase "where necessary," § 6 should be construed to mandate constitutional amendment by disclaimer States.  It is their position that § 6 operates not only to grant the consent of the United States to state action inconsistent with the terms of the enabling legislation, but also to establish a distinct procedure to be followed by Enabling Act States.  To support their position, they rely on the language of the proviso and upon certain legislative history of § 6. [Footnote 26]In the alternative, the Tribe and the United States argue that popular amendatory action, if not compelled by the terms of § 6, is mandated by the terms of the Enabling Act of Page 439 U. S. 483 Feb. 22, 1889, ch 180, § 4.  Although they acknowledge that Congress, in § 6, did grant the "consent of the United States" required under the Enabling Act before the State could remove the disclaimer, they contend that § 6 did not eliminate the need for the "consent of the people" specified in the Enabling Act.  In their view, the 1889 Act -- if not Pub.L. 28 -- dictates that constitutional amendment is the only valid procedure by which that consent can be given.The State draws an entirely different message from § 6.  It contends that the section must be construed in light of the overall congressional purpose to facilitate a transfer of jurisdiction to those option States willing to accept the responsibility.  Section 6 was designed, it says, not to establish, but to remove legal barriers to, state action under the authority of Pub.L. 280.  The phrase "where necessary," in its view, is consistent with this purpose.  It would construe the word "appropriately" in the proviso to be synonymous with "where necessary," and the entire section to mean that constitutional amendment is required only if "necessary" as a matter of state law.  The Washington Supreme Court having found that legislative action is sufficient to grant the "consent of the people" to removal of the disclaimer in Art. XXVI of the state constitution, [Footnote 27] the State argues that the procedural Page 439 U. S. 484 requirements of § 6 have been fully satisfied.  It finds the Enabling Act irrelevant, since, in its view, § 6 effectively repealed any federal law impediments in that Act to state assertion of jurisdiction under Pub.L. 280. [Footnote 28]CFrom our review of the statutory, legislative, and historical materials cited by the parties, we are persuaded that Washington's assumption of jurisdiction by legislative action fully complies with the requirements of § 6.  Although we adhere to the principle that the procedural requirements of Pub.L. 280 must be strictly followed, Kennerly v. District Court of Montana, 400 U. S. 423,  400 U. S. 427; McClanahan v. Arizona State Tax Comm'n, 411 U.S. at  411 U. S. 180, and to the general rule that ambiguities in legislation affecting retained tribal sovereignty are to be construed in favor of the Indians, see, e.g., Bryan v. Itasca County, 426 U.S. at  426 U. S. 392, those principles will not stretch so far as to permit us to find a federal requirement affecting the manner in which the States are to modify their organic legislation on the basis of materials that are essentially speculative.  Cf. Board of County Comm'rs v. United States, 308 U. S. 343,  308 U. S. 350-351.  The language of § 6, its legislative Page 439 U. S. 485 history, and its role in Pub.L. 280 all clearly point the other way.We turn first to the language of § 6.  The main clause is framed in permissive, not mandatory, terms.  Had the drafters intended by that clause to require popular amendatory action, it is unlikely that they would have included the words "where necessary."  As written, the clause suggests that the substantive requirement for constitutional amendment must be found in some source of law independent of § 6.  The basic question, then, is whether that requirement can be found in the language of the proviso to § 6 or alternatively in the terms of the Enabling Act.We are unable to find the procedural mandate missing from the main clause of § 6 in the language of the proviso.  That language, in the abstract, could be read to suggest that constitutional amendment is a condition precedent to a valid assumption of jurisdiction by disclaimer States.  When examined in its context, however, it cannot fairly be read to impose such a condition.  Two considerations prevent this reading.  First, it is doubtful that Congress -- in order to compel disclaimer States to amend their constitutions by popular vote -- would have done so in a provision the first clause of which consents to that procedure "where necessary" and the proviso to which indicates that the procedure is to be followed if "appropriate."  Second, the reference to popular amendatory action in the proviso is not framed as a description of the procedure the States must follow to assume jurisdiction, but instead is written as a condition to the effectiveness of "the provisions of" Pub.L. 280.  When it is recalled that the only substantive provisions of the Act -- other than those arguably to be found in § 7 -- accomplish an immediate transfer of jurisdiction to specifically named States, it seems most likely that the proviso was included to ensure that § 6 would not be construed to effect an immediate transfer to the disclaimer group of option States.  The main clause removes a federal law barrier Page 439 U. S. 486 to any new state jurisdiction over Indian country.  The proviso suggests that disclaimer States are not automatically to receive jurisdiction by virtue of that removal.  Without the proviso, in the event that state constitutional amendment were not found "necessary," [Footnote 29] § 6 could be construed as effecting an immediate cession.  Congress clearly wanted all the option States to "obligate and bind" themselves to assume the jurisdiction offered in Pub.L. 280. [Footnote 30]  To Page 439 U. S. 487 be sure, constitutional amendment was referred to as the process by which this might be accomplished in disclaimer States.  But, given the distinction that Congress clearly drew between those States and automatic-transfer States, this reference can hardly be construed to require that process.Before turning to the legislative history, which, as we shall see, accords with this interpretation of § 6, we address the argument that popular amendatory action, if not a requirement of Pub.L. 280, is mandated by the legislation admitting Washington to the Union.  This argument requires that two assumptions be made.  The first is that § 6 eliminated some, but preserved other, Enabling Act barriers to a State's assertion of jurisdiction over Indian country.  The second is that the phrase "where necessary" in the main clause of § 6 was intended to refer to those federal law barriers that had been preserved.  Only if each of these premises is accepted does the Enabling Act have any possible application.Since we find the first premise impossible to accept, we proceed no further.  Admitting legislation is, to be sure, the only source of law mentioned in the main clause of § 6, and might therefore be looked to as a referent for the phrase "where necessary" in the clause.  This reading, however, is not tenable.  It supplies no satisfactory answer to the question why Congress -- in order to give the consent of the United States to the removal of state organic law disclaimer -- would not also have, by necessary implication, consented to the removal of any procedural constraints on the States imposed by the Enabling Acts.  The phrase "[n]otwithstanding the provisions of any Enabling Act" in § 6 is broad -- broad enough to suggest that Congress, when it referred to a possible necessity for state constitutional amendment, did not intend thereby to perpetuate any such requirement in an Enabling Act.  Even assuming that the phrase "consent of the people" in the Enabling Act must be construed to preclude consent by legislative action -- and the Tribe and the United States have offered Page 439 U. S. 488 no concrete authority to support this restrictive reading of the phrase [Footnote 31] -- we think it obvious that in the "notwithstanding" clause of § 6 Congress meant to remove any federal impediments to state jurisdiction that may have been created by an Enabling Act.The legislative history of Pub.L. 280 supports the conclusion that § 6 did not, of its own force, establish a state constitutional amendment requirement, and did not preserve any such requirement that might be found in an Enabling Act.  Public Law 280 was the first jurisdictional bill of general applicability ever to be enacted by Congress.  It reflected congressional concern over the "law and order" problems on Indian reservations and the financial burdens of continued federal jurisdictional responsibilities on Indian lands, Bryan v. Itasca County, 426 U. S. 373.  It was also, however, without question reflective of the general assimilationist policy followed by Congress from the early 1950's through the late 1960's. [Footnote 32]  Page 439 U. S. 489 See H.R.Rep. No. 848, 83d Cong., 1st Sess. (1953).  See also Hearings on H.R. 459, H.R. 3235, and H.R. 3624 before the Subcommittee on Indian Affairs of the House Committee on Interior and Insular Affairs, 82d Cong., 2d Sess. (1952) (hereinafter 1952 Hearings).  The failure of Congress to write a tribal consent provision into the transfer provision applicable to option States, as well as its failure to consult with the tribes during the final deliberations on Pub.L. 280, provide ample evidence of this. [Footnote 33] Page 439 U. S. 490Indeed, the circumstances surrounding the passage of Pub.L. 280 in themselves fully bear out the State's general thesis that Pub.L. 280 was intended to facilitate, not to impede, the transfer of jurisdictional responsibility to the States.  Public Law 280 originated in a series of individual bills introduced in the 83d Congress to transfer jurisdiction to the five willing States which eventually were covered in §§ 2 and 4. [Footnote 34]   H.R.Rep. No. 848, supra.  Those bills were consolidated into H.R. 1063, which was referred to the House Committee on Interior and Insular Affairs for consideration.  Closed hearings on the bills were held before the Subcommittee on Indian Affairs on June 29, and before the Committee on July 15, 1953. [Footnote 35]  During the opening session on June 29, Page 439 U. S. 491 Committee Members, counsel, and representatives of the Department of the Interior discussed various proposals designed to give H.R. 1063 general applicability.  June 29 Hearings 1-22.  It rapidly became clear that the Members favored a general bill.  Ibid.  At this point, Committee counsel noted that several States "have constitutional prohibitions against jurisdiction."  Id. at 23.  There followed some discussion of the manner in which these States should be treated.  On July 15, a version of § 6 was proposed.  July 15 Hearings 6.  After further discussion of the disclaimer problem, the "notwithstanding" clause was added, id. at 9, and the language eventually enacted as § 6 was approved by the Committee that day.  The speed and the context alone suggest that § 6 was designed to remove an obstacle to state jurisdiction, not to create one.  And the discussion at the hearings, which, in essence, were markup sessions, makes this clear. [Footnote 36] Page 439 U. S. 492While some Committee Members apparently thought that § 6 States, as a matter of state law, would have to amend their constitutions in order to remove the disclaimers found there, [Footnote 37]  Page 439 U. S. 493 there is no indication that the Committee intended to impose any such requirement. [Footnote 38]We conclude that § 6 of Pub.L. 280 does not require disclaimer States to amend their constitutions to make an effective acceptance of jurisdiction.  We also conclude that any Enabling Act requirement of this nature was effectively repealed by § 6.  If, as a matter of state law ,a constitutional amendment is required, that procedure must -- as a matter of state law -- be followed.  And if, under state law, a constitutional amendment is not required, disclaimer States must still take positive action before Pub.L. 280 jurisdiction can become effective.  The Washington Supreme Court having determined that, for purposes of the repeal of Art. XXVI of the Washington Constitution, legislative action is sufficient, [Footnote 39] and appropriate state legislation having been enacted, it follows that the State of Washington has satisfied the procedural requirements of § 6.IVWe turn to the question whether the State was authorized under Pub.L. 280 to assume only partial subject matter and geographic jurisdiction over Indian reservations within the state. [Footnote 40] Page 439 U. S. 494The argument that Pub.L. 280 does not permit this scheme of partial jurisdiction relies primarily upon the text of the federal law.  The main contention of the Tribe and the United States is that partial jurisdiction, because not specifically authorized, must therefore be forbidden.  In addition, they assert that the interplay between the provisions of Pub.L. 280 demonstrates that § 6 States are required, if they assume any jurisdiction, to assume as much jurisdiction as was transferred to the mandatory States. [Footnote 41]  Pointing out that 18 U.S.C. § 1151 defines Indian country for purposes of federal jurisdiction as including an entire reservation notwithstanding "the issuance of any patent," they reason that, when Congress, in § 2, transferred to the mandatory States "criminal jurisdiction" over "offenses committed by or against Indians in the Indian country," it meant that all parts of Indian country were to be covered.  Similarly, they emphasize that civil jurisdiction of comparable scope was transferred to the mandatory Page 439 U. S. 495 States.  They stress that in both §§ 2 and 4, the consequence of state assumption of jurisdiction is that the state "criminal laws" and "civil laws of . . . general application" are henceforth to "have the same force and effect within . . . Indian country as they have elsewhere within the State."  Finally, the Tribe and the United States contend that the congressional purposes of eliminating the jurisdictional hiatus thought to exist on Indian reservations, of reducing the cost of the federal responsibility for jurisdiction on tribal lands, and of assimilating the Indian tribes into the general state population are disserved by the type of checkerboard arrangement permitted by Chapter 36.We agree, however, with the State of Washington that statutory authorization for the state jurisdictional arrangement is to be found in the very words of § 7.  That provision permits option States to assume jurisdiction "in such manner" as the people of the State shall "by affirmative legislative action, obligate and bind the State to assumption thereof."  Once the requirements of § 6 have been satisfied, the terms of § 7 appear to govern the scope of jurisdiction conferred upon disclaimer States.  The phrase "in such manner" in § 7 means at least that any option State can condition the assumption of full jurisdiction on the consent of an affected tribe.  And here Washington has done no more than refrain from exercising the full measure of allowable jurisdiction without consent of the tribe affected.Section 6, as we have seen, was placed in the Act to eliminate possible organic law barriers to the assumption of jurisdiction by disclaimer States.  The Tribe and the United States acknowledge that it is a procedural, not a substantive, section.  The clause contains only one reference of relevance to the partial jurisdiction question.  This is the phrase "assumption of civil and criminal jurisdiction in accordance with the provisions of this Act."  As both parties recognize, this phrase necessarily leads to other "provisions" of the Act for Page 439 U. S. 496 clarification of the substantive scope of the jurisdictional grant.  The first question then is which other "provisions" of the Act govern.  The second is what constraints those "provisions" place on the jurisdictional arrangements made by option States.The Tribe argues, as an initial matter, that § 7 is not one of the "provisions" referred to by § 6.  It relies in part upon the contrast between the phrase "assumption of civil and criminal jurisdiction" in § 6 and the disjunctive phrase "criminal offenses or civil causes of action" in § 7.  From this distinction between the "civil and criminal jurisdiction" language of § 6 and the optional language in § 7, we are asked to conclude that § 6 States must assume full jurisdiction in accord with the terms applicable to the mandatory States even though § 7 States are permitted more discretion.  We are unable to accept this argument, not only because the statutory language does not fairly support it, but also because the legislative history is wholly to the contrary.  It is clear from the Committee hearings that the States covered by § 6 were, except for the possible impediments contained in their organic laws, to be treated on precisely the same terms as option states. [Footnote 42]Section 6, as we have seen, was essentially an afterthought designed to accomplish the limited purpose of removing any barrier to jurisdiction posed by state organic law disclaimers of jurisdiction over Indians.  All option States were originally treated under the aegis of § 7. [Footnote 43]  The record of the Committee hearings makes clear that the sole purpose of § 6 was to resolve the disclaimer problem. [Footnote 44]  Indeed, to the extent that the Tribe and the United States suggest that disclaimer States stand on a different footing from all other option States, their argument makes no sense.  It would ascribe to Congress an Page 439 U. S. 497 intent to require States that, by force of organic law barriers, may have had only a limited involvement with Indian country to establish the most intrusive presence possible on Indian reservations, if any at all, and, at the same time, an intent to allow States with different traditions to exercise more restraint in extending the coverage of their law.The Tribe and the United States urge that even if, as we have concluded, all option States are ultimately governed by § 7, the reference in that section to assumption of jurisdiction "as provided for in [the] Act" should be construed to mean that the automatic transfer provisions of §§ 2 and 4 must still apply.  The argument would require a conclusion that the option States stand on the same footing as the mandatory States.  This view is not persuasive.  The mandatory States were consulted prior to the introduction of the single state bills that were eventually to become Pub.L. 280.  All had indicated their willingness to accept whatever jurisdiction Congress was prepared to transfer.  This, however, was not the case with the option States.  Few of those States had been consulted, and, from the June 29 and July 15 hearings, it is apparent that the drafters were primarily concerned with establishing a general transfer scheme that would facilitate, not impede, future action by other States willing to accept jurisdiction.  It is clear that the "all or nothing" approach suggested by the Tribe would impede even the most responsible and sensitive jurisdictional arrangements designed by the States.  To find that, under Pub.L. 280, a State could not exercise partial jurisdiction, even if it were willing to extend full jurisdiction at tribal request, would be quite inconsistent with this basic history.The language of § 7, which we have found applicable here, provides, we believe, surer guidance to the issue before us. [Footnote 45]  Page 439 U. S. 498 The critical language in § 7 is the phrase permitting the assumption of jurisdiction "at such time and in such manner as the people of the State shall . . . obligate and bind the State to assumption thereof."  Whether or not "in such manner" is fully synonymous with "to such extent," the phrase is at least broad enough to authorize a State to condition the extension of full jurisdiction over an Indian reservation on the consent of the tribe affected.The United States argues that a construction of Pub.L. 280 which permits selective extension of state jurisdiction allows a State to "pick and choose" only those subject matter areas and geographical parts of reservations over which it would like to assume responsibility.  Congress, we are told, passed Pub.L. 280 not as a measure to benefit the States, but to reduce the economic burdens associated with federal jurisdiction on reservations, to respond to a perceived hiatus in law enforcement protections available to tribal Indians, and to achieve an orderly assimilation of Indians into the general population.  That these were the major concerns underlying the passage of Pub.L. 280 cannot be doubted.  See Bryan v. Itasca County, 426 U.S. at  426 U. S. 379.But Chapter 36 does not reflect an attempt to reap the benefits and to avoid the burdens of the jurisdictional offer made by Congress.  To the contrary, the State must assume total jurisdiction whenever a tribal request is made that it do so.  Moreover, the partial geographic and subject matter jurisdiction that exists in the absence of tribal consent is responsive to the law enforcement concerns that underlay the adoption of Pub.L. 280.  State jurisdiction is complete as to all non-Indians on reservations, and is also complete as to Indians on nontrust lands.  The law enforcement hiatus that preoccupied the 83d Congress has, to that extent, been eliminated.  On trust and restricted lands within the reservations Page 439 U. S. 499 whose tribes have not requested the coverage of state law, jurisdiction over crimes by Indians is, as it was when Pub.L. 280 was enacted, shared by the tribal and Federal Governments.  To the extent that this shared federal and tribal responsibility is inadequate to preserve law and order, the tribes need only request, and they will receive, the protection of state law.The State of Washington, in 1963, could have unilaterally extended full jurisdiction over crimes and civil causes of action in the entire Yakima Reservation without violating the terms of Pub.L. 280.  We are unable to conclude that the State, in asserting a less intrusive presence on the Reservation, while at the same time obligating itself to assume full jurisdictional responsibility upon request, somehow flouted the will of Congress.  A State that has accepted the jurisdictional offer in Pub.L. 280 in a way that leaves substantial play for tribal self-government, under a voluntary system of partial jurisdiction that reflects a responsible attempt to accommodate the needs of both Indians and non-Indians within a reservation, has plainly taken action within the terms of the offer made by Congress to the States in 1963.  For Congress surely did not deny an option State the power to condition its offer of full jurisdiction on tribal consent.VHaving concluded that Chapter 36 violates neither the procedural nor the substantive terms of Pub.L. 280, we turn, finally, to the question whether the "checkerboard" pattern of jurisdiction applicable on the reservations of nonconsenting tribes is, on its face, invalid under the Equal Protection Clause of the Fourteenth Amendment. [Footnote 46]  The Court of Appeals Page 439 U. S. 500 for the Ninth Circuit concluded that it is, reasoning that the land-title classification is too bizarre to meet "any formulation of the rational basis test."  552 F.2d  at 1335.  The Tribe advances several different lines of argument in defense of this ruling.First, it argues that the classifications implicit in Chapter 36 are racial classifications, "suspect" under the test enunciated in McLaughlin v. Florida, 379 U. S. 184, and that they cannot stand unless justified by a compelling state interest.  Second, it argues that its interest in self-government is a fundamental right, and that Chapter 36 -- as a law abridging this right -- is presumptively invalid.  Finally, the Tribe argues that Chapter 36 is invalid even if reviewed under the more traditional equal protection criteria articulated in such cases as Massachusetts Bd. of Retirement v. Murgia, 427 U. S. 307. [Footnote 47]We agree with the Court of Appeals to the extent that its opinion rejects the first two of these arguments and reflects a judgment that Chapter 36 must be sustained against an Equal Protection Clause attack if the classifications it employs "rationally furthe[r] the purpose identified by the State."  Massachusetts Bd. of Retirement v. Murgia, supra at  427 U. S. 314.  It is settled that "the unique legal status of Indian tribes under Page 439 U. S. 501 federal law" permits the Federal Government to enact legislation singling out tribal Indians, legislation that might otherwise be constitutionally offensive.  Morton v. Mancari, 417 U. S. 535,  417 U. S. 551-552.  States do not enjoy this same unique relationship with Indians, but Chapter 36 is not simply another state law.  It was enacted in response to a federal measure explicitly designed to readjust the allocation of jurisdiction over Indians.  The jurisdiction permitted under Chapter 36 is, as we have found, within the scope of the authorization of Pub.L. 280.  And many of the classifications made by Chapter 36 are also made by Pub.L. 280.  Indeed, classifications based on tribal status and land tenure inhere in many of the decisions of this Court involving jurisdictional controversies between tribal Indians and the States, see, e.g., United States v. McBratney, 104 U. S. 621.  For these reasons, we find the argument that such classifications are "suspect" an untenable one.  The contention that Chapter 36 abridges a "fundamental right" is also untenable.  It is well established that Congress, in the exercise of its plenary power over Indian affairs, may restrict the retained sovereign powers of the Indian tribes.  See, e.g., United States v. Wheeler, 435 U. S. 313.  In enacting Chapter 36, Washington was legislating under explicit authority granted by Congress in the exercise of that federal power. [Footnote 48]The question that remains, then, is whether the lines drawn by Chapter 36 fail to meet conventional Equal Protection Clause criteria, as the Court of Appeals held.  Under those criteria, legislative classifications are valid unless they bear no rational relationship to the State's objectives.  Massachusetts Bd. of Retirement v. Murgia, supra at  427 U. S. 314.  State legislation "does not violate the Equal Protection Clause merely because the classifications [it makes] are imperfect."  Page 439 U. S. 502 Dandridge v. Williams, 397 U. S. 471,  397 U. S. 485.  Under these standards, we have no difficulty in concluding that Chapter 36 does not offend the Equal Protection Clause.The lines the State has drawn may well be difficult to administer.  But they are no more or less so than many of the classifications that pervade the law of Indian jurisdiction.  See Seymour v. Superintendent, 368 U. S. 351; Moe v. Salish & Kootenai Tribes, 425 U. S. 463.  Chapter 36 is fairly calculated to further the State's interest in providing protection to non-Indian citizens living within the boundaries of a reservation, while at the same time allowing scope for tribal self-government on trust or restricted lands.  The land-tenure classification made by the State is neither an irrational nor arbitrary means of identifying those areas within a reservation in which tribal members have the greatest interest in being free of state police power.  Indeed, many of the rules developed in this Court's decisions in cases accommodating the sovereign rights of the tribes with those of the States are strikingly similar.  See, e.g., United States v. McBratney, supra; Draper v. United States, 164 U. S. 240; Williams v. Lee, 358 U. S. 217; McClanahan v. Arizona State Tax Comm'n, 411 U. S. 164.  In short, checkerboard jurisdiction is not novel in Indian law, and does not, as such, violate the Constitution.For the reasons set out in this opinion, the judgment of the Court of Appeals is reversed.It is so ordered
U.S. Supreme CourtWashington v. Yakima Indian Nation, 439 U.S. 463 (1979)Washington v. Confederated Bands and Tribes of theYakima Indian NationNo. 77-388Argued October 2, 1978Decided January 16, 1979439 U.S. 463SyllabusSection 6 of Pub.L. 280 authorizes the people of States whose constitutions or statutes contain organic law disclaimers of jurisdiction over Indian country to amend "where necessary" their constitutions or statutes to remove any legal impediment to assumption of such jurisdiction under the Act, notwithstanding the provision of any Enabling Act for the admission of the State, but provided that the Act shall not become effective with respect to such assumption of jurisdiction until the people of the State have appropriately amended their state constitution or statutes, as the case may be.  In § 7 of Pub.L. 280, Congress gave the consent of the United States"to any other State . . . to assume jurisdiction at such time and in such manner as the people of the State shall, by affirmative legislative action, obligate and bind the State to assumption thereof."The State of Washington's Constitution contains a disclaimer of authority over Indian country, and hence the State is one of those covered by § 6.  In 1963, after the Washington Supreme Court, in another case, had held that the barrier posed by the disclaimer could be lifted by the state legislature, the legislature enacted a statute (Chapter 36) obligating the State to assume civil and criminal jurisdiction over Indians and Indian territory within the State, subject only to the condition that, in all but eight subject matter areas, jurisdiction would not extend to Indians on trust or restricted lands unless the affected tribe so requested.  Appellee Yakima Nation, which did not make such a request, brought this action in Federal District Court challenging the statutory and constitutional validity of the State's partial assertion of jurisdiction on its Reservation.  The Tribe contended that the State had not complied with the procedural requirements of Pub.L. 280, especially the requirement that the State first amend its constitution; that, in any event, Pub.L. 280 did not authorize the State to assert only partial jurisdiction within an Indian reservation; and that Chapter 36, even if authorized by Congress, violated the equal protection and due process guarantees of the Fourteenth Amendment.  The Page 439 U. S. 464 District Court rejected both the statutory and constitutional claims, and entered judgment for the State.  The Court of Appeals, while rejecting the contention that Washington's assumption of only partial jurisdiction was not authorized by Congress, reversed, holding that the "checkerboard" jurisdictional system produced by Chapter 36 had no rational foundation, and therefore violated the Equal Protection Clause.Held:1. Section 6 of Pub.L. 280 does not require disclaimer States to amend their constitutions to make an effective acceptance of jurisdiction over an Indian reservation, and any Enabling Act requirement of this nature was effectively repealed by § 6.  Here, the Washington Supreme Court, having determined that, for purposes of the repeal of the state constitutional disclaimer, legislative action is sufficient, and, the state legislature having enacted legislation obligating the State to assume jurisdiction under Pub.L. 280, it follows that the State has satisfied the procedural requirements of § 6.  Pp.  439 U. S. 478-493.2. Once the requirements of § 6 have been satisfied, the terms of § 7 govern the scope of jurisdiction conferred upon disclaimer States.  Statutory authorization for the partial subject matter and geographic jurisdiction asserted by Washington is found in the words of § 7 permitting option States to assume jurisdiction "in such manner" as the people of the State shall, "by affirmative legislative action, obligate and bind the State to assumption thereof."  The phrase "in such manner" means at least that an option State can condition the assumption of full jurisdiction on an affected tribe's consent.  Here, Washington has offered to assume full jurisdiction if a tribe so requests.  The partial jurisdiction asserted on the reservations of nonconsenting tribes reflects a responsible attempt to accommodate both state and tribal interests, and is consistent with the concerns that underlay the adoption of Pub.L. 280.  Accordingly, it does not violate the terms of § 7.  Pp.  439 U. S. 493-499.3. The "checkerboard" pattern of jurisdiction ordained by Chapter 36 is not, on its face, invalid under the Equal Protection Clause.  Pp.  439 U. S. 499-502.(a) The classifications based on tribal status and land tenure implicit in Chapter 36 are not "suspect," so as to require that they be justified by a compelling state interest, nor does Chapter 36 abridge any fundamental right of self-government.  Pp.  439 U. S. 500-501.(b) Chapter 36 is valid as bearing a rational relationship to the State's interest in providing protection to non-Indian citizens living within a reservation, while at the same time allowing scope for tribal self-government on trust or restricted lands, the land-tenure classification being neither an irrational nor arbitrary means of identifying those area Page 439 U. S. 465 within a reservation in which tribal members have the greatest interest in being free of state police power.  Pp.  439 U. S. 501-502.552 F.2d 1332, reversed.STEWART, J., delivered the opinion of the Court, in which BURGER, C.J., and WHITE, BLACKMUN, POWELL, REHNQUIST, and STEVENS, JJ., joined.  MARSHALL, J., filed a dissenting opinion, in which BRENNAN, J., joined, post, p.  439 U. S. 502.