What triggered the start of World War II?

The start of World War II was triggered by Adolf Hitler's invasion of Poland on September 1, 1939. This invasion followed the signing of the German-Soviet Nonaggression Pact, which allowed Hitler to attack Poland without fear of a two-front war, as it included provisions for dividing Poland between Germany and the Soviet Union^[1][3]. The subsequent declarations of war by Britain and France against Germany on September 3, 1939, marked the official beginning of the conflict^[2][3].