World War 1 vs World War 2: Causes, Deaths & Key Differences

World War 1 vs World War 2: WW1 (1914–1918) killed 20 million while WW2 (1939–1945) killed 70–85 million. WW2 expanded to the Pacific and introduced nuclear weapons, fundamentally reshaping the world order. WW1 involved ~30 countries; WW2 involved 60+. WW2 introduced nuclear weapons, the Holocaust, and shaped the United Nations and NATO. The Treaty of Versailles after WW1 directly caused WW2 by economically crushing Germany and enabling Hitler's rise.