I think you're playing with semantics. The United States acted in self -defense of its own security and did so in a manner consistent with its stated obligations in that region.
Letter from President Kennedy, December 14, 1961
Dear Mr. President:
I have received your recent letter in which you described so cogently the dangerous condition caused by North Vietnam's efforts to take over your country. The situation in your embattled country is well known to me and to the American people. We have been deeply disturbed by the assault on your country. Our indignation has mounted as the deliberate savagery of the Communist program of assassination, kidnapping, and wanton violence became clear.
Your letter underlines what our own information has convincingly shown - that the campaign of force and terror now being waged against your people and your government is supported and directed from the outside by the authorities at Hanoi. They have thus violated the provisions of the Geneva Accords designed to ensure peace in Vietnam and to which they are bound themselves in 1954.
At that time, the United States, although not a party to the Accords, declared that it "would view any renewal of the aggression in violation of the Agreements with grave concern and as seriously threatening international peace and security." We continue to maintain that view.
In accordance with that declaration, and in response to your request, we are prepared to help the Republic of Vietnam to protect its people and to preserve its independence. We shall promptly increase our assistance to your defense efort as well as help relieve the destruction of the floods which you describe. I have already given the orders to get those programs underway.
The United States, like the Republic of Vietnam, remains devoted to the cause of peace and our primary purpose is to help your people maintain their independence. If the Communist authorities in North Vietnam will stop their campaign to destroy the Republic of Vietnam, the measures we are taking to assist your defense efforts will no longer be necesary. We shall seek to persuade the Communists to give up their attempts of force and subversion. In any case, we are confident that the Vietnamese people will preserve their independence and gain the peace and prosperity for which they have fought so hard and so long.
President Johnson's address to Congress, August 5, 1964
Last night I announced to the American people that the North Vietnamese regime had conducted further deliberate attacks against U.S. naval vessels operating in international waters, and I had therefore directed air action against gunboats and supporting facilities used in these hostile operations. This air action has now been carried out with substantial damage to the boats and facilities. Two U.S. aircraft were lost in the action. After consultation with the leaders of both parties in the Congress, I further announced a decision to ask the Congress for a resolution expressing the unity and determination of the United States in supporting freedom and in protecting peace in southeast Asia. These latest actions of the North Vietnamese regime has given a new and grave turn to the already serious situation in southeast Asia. Our commitments in that area are well known to the Congress. They were first made in 1954 by President Eisenhower. They were further defined in the Southeast Asia Collective Defense Treaty approved by the Senate in February 1955. This treaty with its accompanying protocol obligates the United States and other members to act in accordance with their constitutional processes to meet Communist aggression against any of the parties or protocol states. Our policy in southeast Asia has been consistent and unchanged since 1954. I summarized it on June 2 in four simple propositions: 1. America keeps her word. Here as elsewhere, we must and shall honor our commitments. 2. The issue is the future of southeast Asia as a whole. A threat to any nation in that region is a threat to all, and a threat to us. 3. Our purpose is peace. We have no military, political, or territorial ambitions in the area. 4. This is not just a jungle war, but a struggle for freedom on every front of human activity. Our military and economic assistance to South Vietnam and Laos in particular has the purpose of helping these countries to repel aggression and strengthen their independence. The threat to the free nations of southeast Asia has long been clear. The North Vietnamese regime has constantly sought to take over South Vietnam and Laos. This Communist regime has violated the Geneva accords for Vietnam. It has systematically conducted a campaign of subversion, which includes the direction, training, and supply of personnel and arms for the conduct of guerrilla warfare in South Vietnamese territory. In Laos, the North Vietnamese regime has maintained military forces, used Laotian territory for infiltration into South Vietnam, and most recently carried out combat operations - all in direct violation of the Geneva Agreements of 1962. In recent months, the actions of the North Vietnamese regime have become steadily more threatening... As President of the United States I have concluded that I should now ask the Congress, on its part, to join in affirming the national determination that all such attacks will be met, and that the United States will continue in its basic policy of assisting the free nations of the area to defend their freedom. As I have repeatedly made clear, the United States intends no rashness, and seeks no wider war. We must make it clear to all that the United States is united in its determination to bring about the end of Communist subversion and aggression in the area. We seek the full and effective restoration of the international agreements signed in Geneva in 1954, with respect to South Vietnam, and again in Geneva in 1962, with respect to Laos... 2. Joint Resolution of Congress H.J. RES 1145 August 7, 1964 (Department of State Bulletin, August 24, 1964) Resolved by the Senate and House of Representatives of the United States of America in Congress assembled, That the Congress approves and supports the determination of the President, as Commander in Chief, to take all necessary measures to repel any armed attack against the forces of the United States and to prevent further aggression. Section 2. The United States regards as vital to its national interest and to world peace the maintenance of international peace and security in southeast Asia. Consonant with the Constitution of the United States and the Charter of the United Nations and in accordance with its obligations under the Southeast Asia Collective Defense Treaty, the United States is, therefore, prepared, as the President determines, to take all necessary steps, including the use of armed force, to assist any member or protocol state of the Southeast Asia Collective Defense Treaty requesting assistance in defense of its freedom. Section 3. This resolution shall expire when the President shall determine that the peace and security of the area is reasonably assured by international conditions created by action of the United Nations or otherwise, except that it may be terminated earlier by concurrent resolution of the Congress.