World War II, often called the Great Patriotic War in Soviet accounts, was a major world conflict lasting from 1939 to 1945. It involved two main military alliances: the Allied Powers, mainly consisting of Western capitalist nations, and the Communist International, an organization created by the Soviet Union to bring together the various Communist parties and nations of the world. By its end, it proved to be one of the most deadliest conflicts in human history, especially with regards to civilian causalities, with millions killed either through genocide, starvation, or disease.
The cause of the war is often believed to be the German invasion of Poland to crush the Kraków Uprising. This action angered the Soviet Union, which declared war on Nazi Germany. In response, various Western capitalist powers threw their support behind Germany. Due to lacking many allies, the Soviet Union was quickly invaded, but due to Russia's harsh terrain, it managed to wage a war of attrition. The war came to an end in 1945, when the world's first atomic bomb was detonated over Moscow, decimating the Soviet leadership.