What event led to the start of World War II in Europe?

The invasion of Poland by Germany is widely recognized as the event that marked the beginning of World War II in Europe. On September 1, 1939, Germany launched a military campaign against Poland using a strategy known as Blitzkrieg, characterized by rapid and coordinated attacks using infantry, tanks, and air support. This invasion prompted Britain and France to declare war on Germany on September 3, 1939, signaling the official start of the conflict in Europe.

The context of the event is crucial; prior to the invasion, tensions in Europe had been escalating due to aggressive expansionist policies by Nazi Germany, as well as unresolved issues stemming from the Treaty of Versailles, but these alone did not trigger the war. The invasion of Poland acted as the immediate catalyst, leading directly to a wider war as countries were drawn into the conflict in response.