World War II erupted from a volatile mix of nationalism, militarism, and territorial ambitions. Germany, Italy, and Japan, driven by extreme ideologies and expansionist goals, challenged the global order. Their aggressive actions set the stage for a conflict that would reshape the world.

The United States initially stayed out of the war, clinging to isolationism. But as the conflict escalated, America gradually shifted towards intervention. The Japanese attack on Pearl Harbor in 1941 finally propelled the U.S. into full-scale war against the Axis powers.

Causes of World War II

Nationalism, Militarism, Territorial Expansion

• Extreme nationalism took hold in Germany, Italy and Japan after WWI
  • Germany felt humiliated by Treaty of Versailles that stripped its territories and military and sought to reclaim its power and prestige
  • Mussolini's fascist ideology glorified the Italian state as supreme and called for territorial expansion to restore Roman greatness
  • Japan sought to dominate Asia as a colonial power and secure resources through military conquests of neighboring lands
• Militaristic leaders and cultures emerged that promoted war and aggression
  • Hitler and the Nazis rearmed Germany in violation of treaty restrictions on its military size and weapons
  • Japanese military gained increasing control over civilian government in 1930s and saw conquest as way to solve economic problems
• Territorial ambitions and expansionism became driving forces behind aggression
  • Hitler aimed to create a "Greater Germany" by annexing German-speaking lands like Austria and Czechoslovakia's Sudetenland
  • Mussolini invaded Ethiopia in 1935 to build a "New Roman Empire" in Africa and avenge past Italian defeat there
  • Japan invaded Manchuria in 1931 and China in 1937 to expand its empire and establish dominance over East Asia

Appeasement

• Britain and France sought to avoid another war through a policy of appeasement towards German demands
  • Allowed remilitarization of the Rhineland in 1936 that had been demilitarized after WWI
  • Did not intervene when Hitler annexed Austria in 1938 uniting all German-speakers
  • Accepted German demands on Czechoslovakia at 1938 Munich Conference ceding Sudetenland to avoid war
• Appeasement was based on several factors and assumptions about Hitler's intentions
  • Belief that some of Germany's grievances about Versailles treaty were justified and should be addressed
  • Fear that confronting Hitler militarily would lead to war which they were unprepared for so soon after WWI
  • Hope that Hitler could be satisfied by limited concessions and would not demand more territory
• Policy of appeasement failed to contain Hitler's ambitions and aggression and made war more likely
  • Emboldened Hitler and convinced him that Western powers were weak and would not fight his expansion
  • Led Hitler to believe he could invade Poland in 1939 to gain Lebensraum without facing major opposition
  • Gave Germany time to build up its military strength and power while Allies stayed passive

U.S. Entry into World War II

Isolationism to Intervention

• U.S. was initially isolationist and reluctant to get involved in WWII to avoid another entanglement
  • Neutrality Acts of 1930s aimed to avoid entanglement in foreign wars by banning arms sales and loans to belligerents
  • Many Americans felt burned by WWI's costs and propaganda and wanted to focus on domestic issues like Great Depression
• Roosevelt started to move away from strict neutrality to aid Allies as war escalated
  • "Cash and carry" policy allowed Allies to buy U.S. arms in 1939 as long as they transported them themselves
  • Lend-Lease Act in 1941 allowed U.S. to lend military supplies to Allies to keep them fighting without direct involvement
• Growing tensions with Japan over its aggression in Asia and threat to U.S. interests there
  • U.S. opposed Japanese invasion of China in 1937 and Indochina in 1940 as violations of Open Door Policy
  • Imposed economic sanctions and oil embargo on Japan in 1941 to deter further expansion and force withdrawal
• Japanese attack on Pearl Harbor in December 1941 led U.S. to fully enter war against Axis
  • Over 2,400 Americans killed in surprise attack on naval base in Hawaii seen as "date which will live in infamy"
  • U.S. declared war on Japan; Germany and Italy then declared war on U.S. as Axis allies
• Marked major shift as U.S. mobilized its full industrial and military might for total war against the Axis powers