The war is generally considered to have begun on 1 September 1939, with the invasion of Poland by Nazi Germany and subsequent declarations of war on Germany by France and most of the countries of the British Empire and Commonwealth. Many countries were already at war by this date, such as Ethiopia and Italy in the Second Italo-Abyssinian War and China and Japan in the Second Sino-Japanese War.^[3] Many that were not initially involved joined the war later in response to events such as the German invasion of the Soviet Union and the Japanese attacks on the U.S. Pacific Fleet at Pearl Harbor and on British overseas colonies, which triggered declarations of war on Japan by the United States, the British Commonwealth,^[4] and the Netherlands.^[5]
The war ended with the victory of the Allies in 1945, leaving the political alignment and social structure of the world significantly changed. While the United Nations was established to foster international cooperation and prevent future conflicts, the Soviet Union and the United States emerged as rival superpowers, setting the stage for the Cold War, which lasted for the next forty-six years. Meanwhile, the acceptance of the principle of self-determination accelerated decolonization movements in Asia and Africa, while Western Europe began moving toward economic recovery and increased political integration.