After World War I, the major European experienced two decades of relative peace. The Russian Civil War lasted from 1917 to 1922, saw the intervention of various European powers, ending in victory for the Bolsheviks and the formation of the USSR. Simultaneously, the emerging Soviet government conducted the Polish-Soviet War that lasted from 1919 to 1921 and an ongoing conflict with Finland. The Irish War of Independence was fought from 1919-1921, a guerrilla war against the United Kingdom. It was followed by the Irish Civil War, which erupted over a disagreement between nationalist factions about the Anglo-Irish Treaty, establishing Ireland as a dominion of the British Empire.