The leading cause of World War II, according to historians, is often attributed to the aggressive expansionist policies of Nazi Germany, under Adolf Hitler, intertwined with the failures of the Treaty of Versailles. The treaty, which concluded World War I, imposed harsh reparations and territorial losses on Germany, creating a climate of resentment and economic hardship. This environment facilitated the rise of extremist ideologies and authoritarian regimes. Additionally, Hitler's invasion of Poland in September 1939 is frequently seen as the immediate trigger for the war, as it led to Britain and France declaring war on Germany, escalating the conflict globally.