Correct answer: Russia's withdrawal from the war (World War I).
Russia withdrew from The Great War (which we now call World War I) after the Bolshevik Revolution brought Vladimir Lenin and the communists to power. Revolutionary activity broke out in Russia in February, 1917 -- during the time of World War I. Germany and Russia were on opposite sides in that war, and the German authorities helped Lenin, in exile, return to Russia in order to foment further revolution there and hurt the Russian government's ability to wage war in Europe. The major revolution in Russia, known as the Bolshevik Revolution, occurred in the fall of 1917. In March 1918, the new Soviet Russia government under Lenin ended its participation in the war, signing the Treaty of Brest-Litovsk.