World War II began on September 1, 1939, when Germany invaded Poland. This invasion prompted Britain and France to declare war on Germany, marking the official start of the conflict. The war quickly expanded as alliances formed and nations were drawn into the battle. It ultimately involved many of the world's nations and became one of the largest and deadliest wars in history. The war continued until 1945, ending with the unconditional surrender of Germany in May and Japan in September. The consequences of World War II were profound, shaping global politics, economies, and societies in the years that followed.