The main reason why the United States lost the war in Vietnam was that the popular support for the US involvement in that armed conflict had been eroded, due to the media influence, especially after the Tet Offensive launched by the North Vietnamese and Vietcong forces on January 31, 1968. Although it seemed at the beginning that this wave of surprise attacks on more than 100 objectives in South Vietnam was a communist victory, the majority of the attacks of the Tet Offensive had been successfully repelled by the American and South Vietnamese troops, who would go on the counter-offensive right afterwards; many North Vietnamese Army and Vietcong units had been completed mauled as they left behind thousands of casualties; the enemy had received a hard blow. However, the way the American press narrated and portrayed these armed events wrongly convinced the American public that the United States were losing the war and that any further escalation of the conflict was futile, triggering a nation-wide wave of peace demonstrations.
As a consequence of the negative public opinion pressure and of the Democratic Primary election result in New Hampshire, President Lyndon B Johnson announced in a live speech on March 31, 1968, that he had decided to suspend the bombing of North Vietnam and try to reach a peaceful solution of the armed struggle at the negotiating table as General Westmoreland announced the cessation of counter-offensive operations, which was a big mistake from a military point of view. American Congress also cut back the war funds in respond to the anti-war demonstrations. The war had become so unpopular that year, that in that live speech of March 31, Lyndon B. Johnson also announced that he would not run for a second term in Office. Thus, in the November 5 presidential elections, the Republican candidate Richard Nixon was elected President of the United States. The new administration decided to implement a new policy in South Asia; the “vietnamization of the war”, which consisted in the gradual withdrawal of American ground troops of South Vietnam, to be replaced by South Vietnamese Army units after these have been thoroughly trained and equipped.
To sum up, there were three immediate causes or reasons why the USA lost the war: lack of public support, the sharp cutting back of the war budget, and de-escalation of the conflict by drawing off troops from the theater of operations. But there was a fourth cause; the Cold War context in which the war broke out. The United States went to South Asia to support the government of South Vietnam against the Vietcong attacks and North Vietnamese encroachment and infiltration and thus avoid a domino effect, another western-leaned Asian country falling in Communist hands. However, not only the domino effect fear was part of that historical context, but also a world-wide ideological and military escalation of a low intensity conflict against a guerrilla group; this meant a Chinese direct military envolvement as it had happened during the Korean War, but this time it had to be avoided at all cost, since a Chinese envolvement would mean a Third World War as they had already got their nuclear weapons. That was why, from the very beginning, the United States did not go to South East Asia to fight a total war to defeat North Vietnam, but a limited undeclared one against a scurrying and elusive enemy.