During WW1 the US had no true army so issued a draft, recruited and deployed troops but during most of WW1 US troops were used as replacements for fallen troops, of the UK, France and the other Allies
WW2 was a world power and and fought longer and harder then the French, and the UK. We took out Japan by are selves and, helped take out Germany, an Italy.
Vietnam were just backup for the South Vietnam because North was a communist, and the South was a democracy, or a mixed. At that time we were still trying to stop the spread of communism. So we helped to protect southern towns and civil ans while we fought and trained the south troops.
