The Vietnam War is often regarded as a low point for the U.S. Army due to the use of drafted personnel, the unpopularity of the war with the American public, and frustrating restrictions placed on the military by American political leaders. While American forces had been stationed in the Republic of Vietnam since 1959, in intelligence & advising/training roles, they did not deploy in large numbers until 1965, after the Gulf of Tonkin Incident. American forces effectively established and maintained control of the "traditional" battlefield, however they struggled to counter the guerrilla hit and run tactics of the communist Viet Cong and the North Vietnamese Army. On a tactical level, American soldiers (and the U.S. military as a whole) did not lose a sizable battle.
If it is possible to answer this question, answer it for me (else, reply "unanswerable"): What caused American forces to deploy in large number in 1965?
Gulf of Tonkin Incident