Which factor played the most significant role in the start of world war 2?

The most significant factor in the start of World War II was the aggressive expansionist policies of Nazi Germany under Adolf Hitler. The invasion of Poland on September 1, 1939, marked the official beginning of the war, driven by Hitler's ambitions to expand German territory and assert dominance in Europe. Additionally, the failure of the Treaty of Versailles to establish lasting peace and the policy of appeasement adopted by Western powers contributed to the conditions that allowed for the conflict to escalate.