World War II began mainly as a result of Nazi Germany reoccupying increasing amounts of land over which, according to the terms of the Treaty of Versailles, it no longer had claim. England and France, allied countries that fought against Germany in World War I, declared war on Germany on Sept. 3, 1939, after it invaded Poland, despite repeated demands to cease military occupation.
Germany had been hit hard by the end of World War I because it faced sole responsibility for repaying the costs of the war. To expand the available land for population, Germany re-established control over territories it had owned prior to the war, and some areas that it did not. England and France were the two main allied forces at the start of the war. In 1941, Germany attacked the Soviet Union, violating the Molotov-Ribbentrop Pact and ending the state of nonaggression between the two countries, bringing Russia into the war against Germany.
Meanwhile, Japan had been gathering strength and resources in the Pacific. To halt American intervention, the Japanese government ordered a strike on Pearl Harbor, which took place on Dec. 7, 1941. The United States, which had previously been neutral, declared war on Japan the following day.