The event that sparked the start of World War II in Europe in 1939 was the invasion of Poland by Germany on September 1. This aggression violated several international treaties and prompted Britain and France to declare war on Germany. The invasion was characterized by the Blitzkrieg military strategy, which combined rapid assaults by air and ground forces, leading to a swift defeat of Polish defenses. This marked not only the beginning of a devastating conflict in Europe but also showcased the failure of appeasement strategies previously adopted by Western powers to avoid confrontation with Adolf Hitler's expanding ambitions.