The event that led to the start of World War II in Europe was the invasion of Poland by Nazi Germany on September 1, 1939. This aggressive act violated international agreements and prompted Britain and France to declare war on Germany, marking the beginning of the conflict. The invasion was part of Adolf Hitler's expansionist policy, fueled by a desire for territorial gain and a return to German pre-World War I boundaries. The rapid success of the Blitzkrieg tactics showcased Germany’s military prowess, further escalating hostilities across Europe and setting the stage for a widespread and devastating war.