The event that sparked the start of World War II in Europe was the invasion of Poland by Nazi Germany on September 1, 1939. This aggressive act violated the Treaty of Versailles and was the culmination of Adolf Hitler's expansionist policies aimed at annexing territories for Germany. The invasion prompted Britain and France to declare war on Germany on September 3, 1939, marking the official beginning of the conflict. This event set off a series of military operations that would unfold across Europe and eventually lead to a global confrontation involving numerous nations, fundamentally reshaping the world order.