The primary cause for the outbreak of World War II was the Treaty of Versailles, which ended World War I but left Germany economically devastated and politically unstable. The harsh reparations imposed created widespread resentment, leading to the rise of Adolf Hitler and the Nazis. Militarization and expansionist policies initiated by Germany, Italy, and Japan culminated in invasion and aggression throughout Europe and Asia, triggering a global conflict. The failure of the League of Nations to maintain peace further exacerbated the situation.