The event that marked the beginning of World War II was the invasion of Poland by Germany on September 1, 1939. This aggressive act led to Britain and France declaring war on Germany, as they had commitments to defend Poland. The invasion showcased the failure of appeasement strategies used by European powers in the preceding years and marked a significant escalation in international tensions post-World War I. This conflict would rapidly expand to involve many nations and become one of the deadliest wars in history, lasting until 1945.