The event that marked the beginning of World War II was the invasion of Poland by Germany on September 1, 1939. This surprise attack prompted Britain and France to declare war on Germany on September 3, 1939, officially starting the conflict. The invasion was a culmination of aggressive expansionist policies by Nazi Germany under Adolf Hitler and demonstrated a blatant disregard for the Treaty of Versailles. The swift and coordinated military strategy known as Blitzkrieg overwhelmed Polish defenses, leading to the rapid occupation of Poland and setting the stage for further escalations in Europe.