The event that preceded the start of World War II in Europe was the German invasion of Poland on September 1, 1939. This aggressive act was a direct violation of the Treaty of Versailles and led Britain and France to declare war on Germany on September 3, 1939. The invasion was characterized by the "Blitzkrieg" tactic, which involved rapid and coordinated attacks by air and ground forces. This marked the official beginning of hostilities in Europe, setting off a catastrophic global conflict that would last until 1945 and result in millions of deaths and widespread destruction.