Legality of the Vietnam War

The legality of the Vietnam War refers to the lawfulness of the events from 1965-1975 U.S. military activity that occurred in Vietnam.
Under U.S. law.
The Gulf of Tonkin Resolution, passed in 1964, authorized U.S. President Lyndon Johnson to use military force in Southeast Asia. The resolution was repealed in 1971, but President Richard Nixon cited his power as commander-in-chief of U.S. forces under as the legal authority for operations in Vietnam. No formal declaration of war was ever made, which was, according to many, a violation of the U.S. Constitution
Under international law.
Chapter VII of the United Nations Charter prohibits war except to maintain or to restore international peace (Article 42) or in self-defense (Article 51). According to Richard Falk, "If the US Government had abided by international law, the dreadful experience of the Vietnam War would not have occurred." However, according to Benjamin B. Ferencz,
Legal action.
In United States v. Sisson, a federal judge dismissed a challenge to the Vietnam War's constitutionality because it involved "just the sort of evidence, policy considerations, and constitutional principles which elude the normal processes of the judiciary and which are far more suitable for determination by coordinate branches of the government."
In Sarnoff v. Shultz, the U.S. Ninth Circuit Court ruled on, and the U.S. Supreme Court was petitioned to reconsider, the constitutionality of then Treasury Secretary, George Schultz, allocating funds to the Vietnam War in spite of the fact that an official Declaration of War had never been made. The Supreme Court voted 6-3 to not reconsider the lower court's ruling, thus upholding Ninth Circuit's ruling that George Shultz had been tacitly authorized by the U.S. Congress to allocate funds to the Vietnam War by virtue of Congress's approval of the federal budgets which included these war funds.
References.
3. United States Supreme Court SARNOFF v. SHULTZ, (1972) No. 71-1652 Argued:N/A Decided: October 16, 1972 