a history of the united states involvement in world war two Reluctant warriors: the united states in world war i for the united states, world war i was a short war the united states did not join the allies in their war against the central powers until april 6, 1917, thirty-two months after the war began, and us troops did not see action until well into 1918 then, just a few months after america's active entry into the war, the war was over.