What major event triggered the start of World War II?

The invasion of Poland by Germany on September 1, 1939, is widely recognized as the major event that triggered the start of World War II. This military action followed a period of escalating aggression by Nazi Germany under Adolf Hitler, who sought to revise the post-World War I order established by the Treaty of Versailles. The invasion of Poland led Britain and France to declare war on Germany on September 3, 1939, marking the official beginning of the conflict in Europe.

As for other choices, while the assassination of Archduke Franz Ferdinand was a catalyst for World War I, it is not related to the events leading up to World War II. The bombing of Pearl Harbor, which occurred on December 7, 1941, was a significant attack that led to the United States entering the war, but it was not the event that started it. Similarly, although the signing of the Treaty of Versailles had long-lasting effects that contributed to the conditions leading to World War II, it was not a singular event that initiated the war itself. The direct military aggression exhibited in Poland clearly marked the escalation from regional conflicts to a global war.