Who Says We Never Strike First? And the United States entered Vietnam not to avenge two attacks on American warships in the Gulf of Tonkin (one of which didn't occur) but because President Lyndon B. Johnson wanted to prevent the spread of Communism. This is not to suggest that the United States was necessarily wrong to enter these wars; the Mexican War, Spanish-American War and Vietnam War all had large elements of moral purpose. The point is simply that we have often sought out battle, not waited for it to come to us. Many such interventions have been undertaken as part of America's long-standing commitment to act as a global policeman.

I think it says something about how different the premises of the pro- and anti-war factions are that someone could use Vietnam and sundry other US interventions during the Cold War as evidence of why preemptive strikes have been beneficial in the past.