Germany's biggest rivals/enemies were the Triple Entente, and in particular Britain and France, because they were more stable and had more military power than Russia, who were the other main force in the Triple Entente.
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The US joined the war on the side of the Allies on April 6, 1917.
Britain, US, Russia, Canada, Australia.
Allies
Quite simply put, they didn't. The U.S entered World War One, to late to actually make a significant difference.
World war 1 ended because the Germans surrended to the Allies