World War II, a catastrophic conflict, reshaped the world's geopolitical landscape and economy, claiming millions of lives and challenging global powers. Lasting six years from 1939 to 1945, it involved numerous nations in violent confrontation. Root causes included repercussions of the Treaty of Versailles post-World War I, the Great Depression, failed appeasement policies, and the rise of militarism in Germany and Japan, compounded by the League of Nations' ineffectiveness. Widely regarded as the deadliest conflict in history, it resulted in approximately 85 million casualties among military personnel and civilians worldwide, leaving an indelible mark on history and international relations.