What event is often cited as the catalyst for the start of World War II?

The invasion of Poland is identified as the catalyst for the start of World War II because it marked the point at which diplomatic tensions escalated into full-scale conflict. On September 1, 1939, Germany invaded Poland using a strategy known as Blitzkrieg, or "lightning war," which involved fast and overwhelming attacks combining air and ground forces. This action prompted Britain and France to declare war on Germany on September 3, 1939, as they had previously pledged to support Poland in the event of an invasion.

This invasion was significant not only because it triggered declarations of war but also because it demonstrated the failure of diplomatic efforts to contain German aggression. The earlier events listed, such as the sinking of the Lusitania and the assassination of Archduke Francis Ferdinand, while impactful in their historical contexts, did not lead to the immediate outbreak of World War II. The bombing of Pearl Harbor, on the other hand, was a critical event in the United States' entry into the existing conflict, but it occurred after the war had already begun with Germany's invasion of Poland. Thus, the invasion of Poland serves as the definitive moment that ignited the larger conflict.