The nation that joined the war in 1917 was the United states but not necessarily as part of the allies, rather America had opted to fight Germany, an enemy of the allies, more as an associate ally to protect its own interests.
American went to war to stop German aggression and spread values of democratic freedom and made decisive contributions to the final battles of 1918.
The Americans decided neutrality would not do when German aggression on the high seas went on to such an extent that it threatened U.S. national security, and so the Germans had to be stopped.
