Russia was a major participant in World War I, entering the war in 1914 on the side of the Allied Powers (the Entente) against the Central Powers. The country’s involvement was marked by initial military actions on the Eastern Front and severe internal consequences, including economic crisis and mass casualties, which ultimately led to the February Revolution of 1917 and the overthrow of Tsar Nicholas II. Russia officially withdrew from the war in 1918 after signing the Treaty of Brest-Litovsk.The United States entered World War I on April 6, 1917.
WWII was an alliance of convenience. Germany declared war on the U.S. on Dec 11, 1941. The Soviet Union was a communist country, which did not share our values. Stalin was a dictator. When the war was over, the alliance ceased to exist. The Soviets became an adversary almost before the war was over. The negotiations at Yalta were not friendly discussions among allies.