World War II was waged between the Allied forces and the Axis forces in the years 1939 to 1945. The first use of the atomic bomb was instrumental in determining the outcome of the war.

World War II began on August 31, 1939, when Germany, under Adolph Hitler, invaded Poland. As a result, Great Britain and France declared war on Germany on September 3. Soviet troops invaded Poland's eastern border on September 17, and Germany and the Soviet Union agreed to divide a defeated Poland between them. By October 10, Soviet forces easily established themselves in Estonia, Latvia, and Lithuania. Meanwhile, skirmishes between British naval forces and German U-boats (submarines) took place in September and October of that year.

In February 1940, the Soviet Union attacked Finland, achieving victory by March 6. In April, Germany successfully invaded and occupied both Denmark and Norway. In May, Germany successfully invaded and occupied Belgium. From there, German...