What event triggered the start of World War II?

The invasion of Poland by Germany in 1939 is recognized as the event that triggered the start of World War II primarily because it marked the moment when aggressive expansionism in Europe escalated into widespread conflict. On September 1, 1939, Germany launched its invasion of Poland using a strategy known as Blitzkrieg, or "lightning war," which emphasized rapid and coordinated assaults by air and ground forces. This invasion prompted Britain and France to declare war on Germany on September 3, 1939, signaling the initiation of World War II.

This event is significant because it demonstrated not only Germany's blatant disregard for the sovereignty of other nations but also the failures of appeasement policies that had been employed by other European powers in the years leading up to the war. The invasion of Poland thus stands as the pivotal act that transformed tensions in Europe into a full-scale global conflict.