https://www.mtholyoke.edu/acad/intrel/pentagon4/ps11.htm
Legal Memorandum Prepared by Leonard C. Meeker, State Department Legal Advisor, for Submission to the Senate Committee on Foreign Relations, March 4, 1966, "The Legality of United States Participation in the Defense of Viet-Nam"; Department of State Bulletin, March 28, 1966, pp. 15-16
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"V. CONCLUSION
"South Viet-Nam is being subjected to armed attack by Communist North Viet-Nam, through the infiltration of armed personnel, military equipment, and
regular combat units. International law recognizes the right of individual and collective self-defense against armed attack. South Viet-Nam, and the United States upon the request of South Viet-Nam, are engaged in such collective defense of the South. Their actions are in conformity with international law and with the Charter of the United Nations. The fact that South Viet-Nam has been precluded by Soviet veto from becoming a member of the United Nations and the fact that South Viet-Nam is a zone of a temporarily divided state in no way diminish the right of collective defense of South Viet-Nam.
"The United States has commitments to assist South Viet-Nam in defending itself against Communist aggression from the North. The United States gave undertakings to this effect at the conclusion of the Geneva conference in 1954. Later that year the United States undertook an international obligation in the SEATO treaty to defend South Viet-Nam against Communist armed aggression. And during the past decade the United States has given additional assurances to the South Vietnamese Government.
"The Geneva accords of 1954 provided for a cease-fire and regroupment of contending forces, a division of Viet-Nam into two zones, and a prohibition on the use of either zone for the resumption of hostilities or to 'further an aggressive policy.' From the beginning, North Viet-Nam violated the Geneva accords through a systematic effort to gain control of South Viet-Nam by force. In the light of these progressive North Vietnamese violations, the introduction into South Viet-Nam beginning in late 1961 of substantial United States military equipment and personnel, to assist in the defense of the South, was fully justified; substantial breach of an international agreement by one side permits the other side to suspend performance of corresponding obligations under the agreement. South Viet-Nam was justified in refusing to implement the provisions of the Geneva accords calling for reunification through free elections throughout Viet-Nam since the Communist regime in North Viet-Nam created conditions in the North that made free elections entirely impossible.
"The President of the United States has full authority to commit United States forces in the collective defense of South Viet-Nam. This authority stems from the constitutional powers of the President. However, it is not necessary to rely on the Constitution alone as the source of the President's authority, since the SEATO treaty-advised and consented to by the Senate and forming part of the law of the land-sets forth a United States commitment to defend South Viet-Nam against armed attack, and since the Congress--in the joint resolution of August 10, 1964, and in authorization and appropriations acts for support of the U.S. military effort in Viet-Nam--has given its approval and support to the President's actions. United States actions in Viet-Nam, taken by the President and approved by the Congress, do not require any declaration of war, as shown by a long line of precedents for the use of United States armed forces abroad in the absence of any congressional declaration of war."
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