Pentagon's Top Lawyer to Resign Jeh C. Johnson, the Obama administration's top lawyer at the Pentagon, will step down at the end of December, the military said on Thursday. Over the course of President Obama's first term, Mr. Johnson was a central player in some of the most important legal policy deliberations of the administration. Mr. Johnson, who was general counsel to the Army during the Clinton administration, was a key legal adviser and fund-raiser for then-Senator Obama during his run for the presidency in the 2008 campaign. On Thursday, he sent Mr. Obama a letter saying that he would resign effective midnight on Dec. 31. "Thank you for the opportunity to be part of your campaign, your transition, and your Administration," Mr. Johnson wrote. Thank you also for the best clients I will ever have: Robert Gates, Leon Panetta, and the men and women of the U.S. military. Mr. Johnson, a former prosecutor, has been mentioned as a potential attorney general should Eric H. Holder Jr. step down in Mr. Obama's second term. That speculation has been centered more among his colleagues in the Pentagon rather than among civilian law enforcement officials, however. In his current job, Mr. Johnson worked closely on internal debates about the scope and limits of the government's power to hold terrorism suspects in indefinite detention and to target them with drone strikes in places like Yemen and Somalia. In those debates he generally sought broader latitude for the government than some others, notably State Department officials. But Mr. Johnson took a more restrained position than some colleagues during the NATO-led air war in Libya. As American participation in the effort neared an apparent 60-day limit imposed by the War Powers Resolution for hostilities that had not been authorized by Congress, he urged pulling back on direct combat activities - like missile strikes - but was overruled by the White House. Mr. Johnson also was closely involved in the decision to overhaul and restart some military commission trials for terrorism suspects, and he helped resolve a dispute over press access to the proceedings in a way that granted reporters greater leeway to cover them. He was an important Pentagon official interacting with Congress as lawmakers imposed steep restrictions on the transfer of detainees from the prison at Guantánamo Bay, Cuba, which contributed to the failure by the administration to follow through on Mr. Obama's vow to close that prison. Mr. Johnson also helped lead the successful effort to end the military's "don't ask, don't tell" policy that had barred gays and lesbians from serving openly in the armed forces. Mr. Johnson was also closely involved in responding to the mass leaking of hundreds of thousands of secret military and diplomatic documents through the antisecrecy organization WikiLeaks. He sent a letter to a lawyer representing WikiLeaks demanding that it delete the materials rather than release any further documents and helped to orchestrate the transfer of the Army private accused of leaking the archives, Bradley Manning, out of the Marines brig at Quantico, Va., amid controversy over his restrictive conditions of confinement there. Mr. Johnson is one of several officials who have delivered prominent public speeches outlining the administration's legal theories in the war on terrorism, including the proposition that American citizens who join Al Qaeda may be targeted for killing. Last week, he delivered a high-profile speech at the Oxford Union in England that raised the prospect that Al Qaeda and its allies would eventually be degraded to the point that the war against them will come to a close -- and with it the legal architecture put in place after the terrorist attacks of Sept. 11, 2001 - even if a few individuals and remnants still claim affiliation with the group. When that happens, he said, difficult legal questions will arise about such matters as whether and for how long the United States may keep holding terrorism detainees without trial as wartime prisoners. Mr. Johnson said he would take some time off and then return to private law practice. He is expected to return to the law firm of Paul, Weiss, Rifkind, Wharton & Garrison, where he formerly was a partner.