The event that marked the start of World War II in Europe was Germany's invasion of Poland on September 1, 1939. This aggressive military action prompted Britain and France to declare war on Germany on September 3, 1939, formally initiating the conflict. The invasion was a culmination of the Nazi regime's expansionist policies under Adolf Hitler and violated the Treaty of Versailles. The Blitzkrieg tactics employed by the German military allowed for swift victories, resulting in the rapid occupation of Poland. This invasion signified the beginning of a larger global conflict, ultimately leading to widespread devastation across multiple continents.