US involvement in the war was a gradual process. This involvement increased over the years under three U.S. Presidents, both Democratic and Republican (successively Eisenhower, Kennedy and Johnson, and was sustained for additional years in the administration of Richard Nixon), despite warnings by the US military leadership against a major ground war in Asia. Though actions under the administrations of Eisenhower and Kennedy are considered to have cast the die for the future conflict, it was Johnson who expanded and transformed the engagement into a distinctly U.S. operation, a policy which eventually led to opposition within his own party that convinced him not to seek a second term in 1968 after internal polling showed the depth of public doubt and anger.

There was never a formal declaration of war but there were a series of presidential decisions that increased the number of "military advisors" and then active combatants in the region.

The prominent linguist and leftist anti-war critic Noam Chomsky claims that Kennedy ordered the US Air Force to start bombing South Vietnam as early as 1962, using South Vietnamese aircraft markings to disguise US involvement. He also accuses Kennedy of authorizing the use of napalm, along with other crop destruction programs at this earlier date, rather than as a later part of the larger war. The traditional view claims that "actual increased U.S. involvement in the Vietnam War" didn't occur until 1964.

The program of covert GVN (South Vietnamese) operations was designed to impose "progressively escalating pressure" upon the North, and initiated on a small and essentially ineffective scale in February 1964, according to standard sources. The active U.S. role in the few covert operations that were carried out was limited essentially to planning, equipping, and training of the GVN forces involved, but U.S. responsibility for the launching and conduct of these activities was unequivocal and carried with it an implicit symbolic and psychological intensification of the U.S. commitment.