What event is widely recognized as the trigger for the start of World War II?

The invasion of Poland by Germany is widely recognized as the key trigger for the start of World War II. This event marked the beginning of large-scale hostilities in Europe, as Germany, under the leadership of Adolf Hitler, invaded Poland on September 1, 1939. The invasion violated existing treaties and agreements, prompting Britain and France to declare war on Germany on September 3, 1939. This act not only crossed a significant threshold of aggression but also demonstrated the failure of appeasement policies and prior diplomatic efforts to contain Hitler's expansionist ambitions.

The attack on Pearl Harbor occurred later, on December 7, 1941, and marked the United States' entry into the war, but it did not trigger the conflict itself. The bombing of Hiroshima happened in 1945, later in the war, and the signing of the Treaty of Versailles took place in 1919, serving as a peace settlement that ended World War I but did not directly instigate World War II. In sum, the invasion of Poland is the pivotal moment that initiated the widespread conflict we recognize as World War II.