What event or series of events triggered world war 2?

World War II was primarily triggered by the aggressive expansionist policies of Nazi Germany under Adolf Hitler, particularly the invasion of Poland on September 1, 1939. This act violated the Treaty of Versailles and prompted Britain and France to declare war on Germany. Additionally, the failure of appeasement strategies, exemplified by the Munich Agreement, and the rise of totalitarian regimes in Italy and Japan contributed to the escalating tensions that led to the global conflict.