The significant event that preceded the start of World War II in Europe was the invasion of Poland by Nazi Germany on September 1, 1939. This invasion marked the official beginning of the war in Europe, as it prompted Britain and France to declare war on Germany. The act was part of Hitler's aggressive expansionist policies and violated the Treaty of Versailles. Poland's swift defeat showcased the effectiveness of new military tactics, such as the Blitzkrieg, and altered the geopolitical dynamics of Europe, leading to widespread conflict.