How did the united states increase in the size of its military during the vietnam war?

I'm pretty sure the army made all eligible adults 18 and older join the army, or they were threatened with imprisonment and sent to jail.

by a variety of things but the 2 most known are the hawks or a person who supported U. S. involvement in the Vietnam War and believed that the United States should use increased military force to win it and by william westmoreland, an american commander in south vietnam continued to request more troops.