What event is often cited as the beginning of World War II?

The invasion of Poland by Nazi Germany in 1939 is widely recognized as the beginning of World War II due to its significant geopolitical implications and the immediate consequences that followed. On September 1, 1939, Germany launched a military attack on Poland, using a strategy known as Blitzkrieg, or "lightning war," which emphasized rapid movement and coordination between air and ground forces. This invasion prompted Britain and France to declare war on Germany on September 3, 1939, marking the official escalation of the conflict into a global war.

This event is critical because it not only triggered direct military engagements but also highlighted the failure of appeasement policies that had been adopted by European powers in the preceding years. The invasion of Poland shattered the political landscape in Europe, demonstrating that aggressive expansionism could not be contained through diplomacy or treaties, thus setting the stage for widespread warfare across multiple continents. The importance of this event in the broader context of the war confirms its status as the starting point of World War II.