Final answer:
The invasion of Poland by Nazi Germany on September 1, 1939, marked the beginning of World War II in Europe, leading to Britain and France's declaration of war against Germany. This followed a series of aggressive moves by Nazi Germany, including the strategy of blitzkrieg.
Step-by-step explanation:
The event that marked the beginning of World War II in Europe was the German invasion of Poland on September 1, 1939. This aggressive act by Nazi Germany was executed with a strategy known as blitzkrieg or "lightning war," which involved a coordinated attack using airpower and ground forces. The Luftwaffe, the German air force, initiated the invasion by bombing airfields, factories, and cities, followed by the rapid advancement of Panzer tanks and troop transports. Despite the brave resistance of Polish forces, Poland was overwhelmed and officially fell to Nazi control on September 27, 1939. The invasion led Britain and France to declare war on Germany, as they had guaranteed Poland's security, thereby igniting the European conflict of World War II.
Leading up to the war, the Nazis had systematically broken away from the Treaty of Versailles and had begun to re-arm Germany. Their expansionist ambitions were evident in their annexation of Austria and the Sudetenland before open hostilities began. The relative calm of the 'Sitzkrieg' or 'Phony War' that followed the invasion of Poland ended with Hitler's subsequent invasions of Denmark and Norway, preparing the stage for further conquest of Western Europe.