For my generation, the realization descended upon us in the 1960s when the military/security complex convinced Americans that if we permitted Vietnamese nationalist leader Ho Chi Minh to unify Vietnam, the dominoes would fall until the Communist World Revolution had us in its grip. This despite the fact that Stalin had killed off the Trotskyist world revolutionaries and declared "Socialism in one country."
Nationalists in the West's colonies, such as Vietnam and Cuba, misinterpreted the talk about making the world safe for self-determination as applying to them. Ho Chi Minh helped the US against Japan during World War 2. His requests for US help for Vietnamese independence from France were cold-shouldered by the Truman administration. He did not turn against the US until Washington turned against him. http://www.historynet.com/ho-chi-minh-and-the-oss.htm
America's participation in the Vietnam War lasted for a decade or thereabouts. The extraordinary carnage and war crimes served no interest other than the power and profit of the military/security complex and the paranoia of the arbiters of US foreign policy.