Russia pulled out of WWI in 1917 because of a revolution that forced the czar to abdicate and threw the country into chaos. Some of that chaos was caused by Russia's involvement in WWI, but there were many other reasons. The end of serfdom marked the beginning of even more struggles for the Russian people. They went from being forced to rent farmland, to being completely landless. This pushed people into the cities and left many jobless. Beginning with an earlier Russian Revolution in 1905, the people wanted moderate guarantees and concessions that would limit the czar's power and make life easier for the citizens.
The Romanov Dynasty had always been unwilling to give up power and Nicholas II eventually seized control back from Duma, the legislative body. With the outbreak of WWI, Nicholas II hoped for the opportunity to raise his approval rating but the country was not ready for three years of war against Germany. Supply and food shortages caused people to protest again and Nicholas II finally agreed to abdicate. The government that was established was still committed to the war and Communists like Vladimir Lenin did not think the government went far enough. In November of 1917, the first Communist government was established. After a Civil War, the Soviet Union fully rejected Capitalism and Lenin became the effective dictator of Russia.