The main causes of World War I were competition among Europe's imperialist, industrialized nations and a failure of diplomacy. The war transformed European and American life, wrecked the economies of Europe, and resulted in the end of several empires in Europe.
Tsarist Russia entered World War I as an absolute monarchy with sharp class divisions between the nobility and the peasants. The Tsar did little to resolve the grievances of workers and peasants. In addition, incompetent administration of the country during the war led to revolution followed by a short-lived provisional government. A second revolution by the Bolsheviks created the communist state that ultimately became the U.S.S.R.
After World War I, the victors established the League of Nations and a mandate system for Germany's former colonies in an attempt to avoid future conflicts. Unfortunately, the League lacked the power to enforce its decisions while the mandate system planted the seeds for future conflicts in the Middle East.
A period of uneven prosperity in the decade following World War I was followed by a worldwide depression in the 1930s. Some causes of the depression included German reparations, high protective tariffs, excessive expansion of credit, and the stock market crash of 1929. Depression weakened Western democracies, making it difficult for them to challenge the rising threat of totalitarianism. Vladimir Lenin established a communist dictatorship in the Soviet Union, and Joseph Stalin continued to rule after Lenin’s death. The Treaty of Versailles worsened the economic and political conditions in Europe and led to the rise of totalitarian regimes in Italy with Mussolini and in Germany with Hitler and the Nazi Party. Japan emerged as a world power after World War I and conducted aggressive imperialistic policies in Asia with its invasion of Korea, Manchuria, and the parts of China. These totalitarian regimes would present significant challenges for the rest of the world in World War II.