Global Conflict (1900–1945)
What you must know
The two world wars turned industrial capacity against populations. World War I's causes are usually grouped as militarism, alliance blocs, imperial rivalry and Balkan nationalism, ignited by the assassination at Sarajevo; its conduct - trenches, machine guns, gas, submarines, blockades - made it a total war that mobilized economies, colonies and propaganda. Its settlement dissolved the Ottoman and Habsburg empires, imposed reparations and war guilt on Germany at Versailles, created weak new states and a League of Nations without American participation, and installed a mandate system that disguised empire as trusteeship. The Russian Revolution of 1917 created the first communist state, and Stalin's five-year plans and collectivization later built industrial power at immense human cost. Economic collapse after 1929 discredited liberal governments and fed fascist and Nazi movements promising national revival through expansion; appeasement at Munich failed to satisfy Hitler, and the Molotov-Ribbentrop Pact temporarily divided eastern Europe. World War II in Asia began with Japan's invasion of China in 1937 and merged with the European war in 1941; it ended with the Holocaust, the firebombing of cities and the atomic bombs. Women entered workforces, colonial soldiers served, and the war's end set up decolonization and the Cold War.
Traps
Versailles alone did not cause World War II; combine unresolved grievance, depression, ideology and failed collective security. Do not date the Asia-Pacific war from 1939. Mandates were not independence. "Total war" means mobilizing civilians and economies, not merely fielding large armies.
Worked example
For a causality question on 1939, write a chain rather than a list: punitive peace and depression → extremist parties gain support → revisionist expansion → appeasement and failed deterrence → war. Explicit links earn credit.