In 1914, some of the most powerful countries of Europe formed rival alliances, with Germany, Austria-Hungary, and Italy in one alliance, and Russia, the U.K., and France in another. When Russia and Austria-Hungary intervened in fighting that broke out in the Balkans, the rest of Europe was plunged into World War I, which lasted from 1914-1918. In the years after World War I, revolution and civil war impacted Russia, Germany, and the remains of Austria-Hungary. Europe as a whole was greatly changed as a result. Russia and Germany became dramatically smaller, and the Russian revolution of 1917 led to the creation of the Soviet Union. In addition, many smaller states appeared.
World War II began in 1939, when Germany's invasion of Poland forced Britain and France to declare war on Germany. By 1942, most of Europe was under the control of Germany and its allies. But by 1945, the German army was weakened by a reviving Soviet Union, and the war ended in spring of 1945.