The primary cause of World War II was the aggressive expansionist policies of Nazi Germany under Adolf Hitler, which stemmed from unresolved grievances from World War I, particularly the harsh conditions imposed by the Treaty of Versailles. Hitler's ambition to reclaim lost territories and establish German dominance contributed to a series of acts of aggression, including the invasion of Poland in 1939, which triggered the war. Additional factors included the rise of totalitarian regimes in Italy and Japan, economic instability during the Great Depression, and the failure of the League of Nations to maintain peace. Together, these elements set the stage for a global conflict that would result in unprecedented destruction.