What was the reason (the trigger) for the USA-VIETNAM war in the 1960's?

The United States became involved in the war for a number of reasons, and these evolved and shifted over time. Primarily, every American president regarded the enemy in Vietnam–the Vietminh; its 1960s successor, the National Liberation Front (NLF); and the government of North Vietnam, led by *Ho Chi Minh–as agents of global communism. U.S. policymakers, and most Americans, regarded communism as the antithesis of all they held dear. Communists scorned democracy, violated human rights, pursued military aggression, and created closed state economies that barely traded with capitalist countries. Americans compared communism to a contagious disease. If it took hold in one nation, U.S. policymakers expected contiguous nations to fall to communism, too, as if nations were dominoes lined up on end. In 1949, when the Communist Party came to power in China, Washington feared that Vietnam would become the next Asian domino. That was one reason for Truman’s 1950 decision to give aid to the French who were fighting the Vietminh,

Vietnam like Korea was a proxy war between the US and USSR. During the cold war both superpowers understood that a full scale war between themselves would be devastating with the winner loosing far more than they could ever hope to gain. They looked at the rest of the world like a chessboard wishing to win over countries to their economic ideologies. The USSR supported communist leaning governments while the US supported capitalist or at least anti-communist governments. There was no real concern for democracy. The US overthrew several democratically elected leftist governments while supporting brutal and corrupt dictators who were anti-communist. It was the economic ideology rather than the political system we most cared about.

because of communists