Because Germany was the common enemy of the Soviet Union, the US and the free French and Poles. The USSR was at peace with Japan until the final months of WWII.
Allied.
Russia
France and Russia.
Russia == Russia was in the war from the beginning. It was Italy that didn't join the war in 1914
Russia
Great Britain , France and Russia
Russia. After the october revolution and the fall of the Czar.
Russia, UK, and US ( even Italy, 2nd W.W., not sure) were the main countries of the allied power.
- England - France - Russia - Serbia - U.S. (joined in 1918)
Allied Forces
Russia concluded a separate peace at Brest-Litovsk, after the Bolsheviks took power.
Russia