What event triggered the start of World War II in Europe?

The invasion of Poland by Germany in 1939 is recognized as the event that triggered the start of World War II in Europe because it marked a significant escalation in aggressive military actions by Germany under Adolf Hitler. On September 1, 1939, Germany launched a rapid invasion of Poland, employing blitzkrieg tactics that combined swift attacks by air and ground forces. This invasion was a direct violation of previous agreements, including the Treaty of Versailles, and it prompted Britain and France to declare war on Germany on September 3, 1939. This declaration formed the basis for the widespread conflict that ensued, ultimately expanding into a global war. This event is crucial in understanding the immediate catalysts for World War II, contrasting with other events like the attack on Pearl Harbor, which, while significant, pertains to the U.S. entry into the war rather than its inception.