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World War II
September 1, 1939 – September 2, 1945· War· Global
Well documentedSolid records, physical evidence or scientific consensus.
The deadliest war in history (1939–1945), between the Axis (Germany, Italy, Japan) and the Allies, led by Britain, the Soviet Union, the US and China.
See it on the History map →What happened
Japan had invaded China in 1937. In Europe, war began when Germany invaded Poland on September 1, 1939. Germany conquered most of Europe, then invaded the Soviet Union in 1941. Japan's attack on Pearl Harbor brought the US in. Battles from Midway to Stalingrad turned the tide by 1943. Germany surrendered in May 1945; Japan surrendered after the atomic bombings and a Soviet invasion of Manchuria, signing on September 2. An estimated 70–85 million people died, most of them civilians.
Why it matters
It left the US and USSR as superpowers, Europe and Korea divided, empires fatally weakened, and nuclear weapons in the world, and it produced the United Nations.
How we know
Enormous archives on all sides, captured German and Japanese records, the Nuremberg and Tokyo trials, wartime film and photography, and millions of survivors' testimonies.
Still debated
Death tolls, especially in China and the Soviet Union, remain estimates; even when the war began (1937 in Asia or 1939 in Europe) depends on framing.
Inside this (8)
Connections (12)
▸Meiji Restoration built the modern military that later fought World War IIMostly agreed
The connection
Meiji reformers built a conscript army and modern navy that beat China and Russia. The 1889 constitution let the army and navy answer to the emperor rather than the cabinet. By the 1930s officers used that independence to drive expansion into Manchuria and China, which led to war with the US and Britain.
When and how
Japan, from conscription in 1873 and the 1889 Meiji Constitution to the Manchurian Incident (1931) and full-scale war in China (1937).
How sure are we?
The Meiji Constitution's text, military records, and the 1931 Mukden Incident, a railway bombing staged by army officers without Tokyo's approval.
Debates and open questions
Historians debate whether Meiji institutions set Japan on the road to militarism, or whether the 1930s path came from later choices and the Depression.
▸Treaty of Versailles helped set the stage for World War IIDebated
The connection
One view: the treaty was harsh enough to humiliate Germany but too weak to keep it down, leaving a resentful power eager to overturn it. Hitler tore it up step by step: rearmament in 1935, the Rhineland in 1936, Austria and the Sudetenland in 1938. Others argue the real problem was the Allies' failure to enforce it.
When and how
Europe, 1919–1939; key steps include the remilitarization of the Rhineland (March 1936) and the Munich Agreement (September 1938).
How sure are we?
That Hitler dismantled the treaty is documented; how much the treaty itself caused the war is a matter of interpretation, argued since Keynes in 1919.
Debates and open questions
Was the war Versailles's fault or Hitler's own aims? Historians such as A. J. P. Taylor and Margaret MacMillan have given very different answers.
▸Great Depression helped cause World War IIMostly agreed
The connection
The Depression broke the 1920s world order. Countries raised tariffs and formed trade blocs, undercutting cooperation. Japan, hit hard by collapsing silk exports, turned to military expansion and seized Manchuria in 1931. In Germany the slump brought Hitler to power. Rearmament then became a way out of unemployment, speeding the march to war.
When and how
Worldwide, 1929–1939; key steps include Japan's seizure of Manchuria (1931), Hitler's appointment (1933) and Italy's invasion of Ethiopia (1935).
How sure are we?
Trade, unemployment and political data, and leaders' own statements tying expansion to economic needs, such as Japan's case for Manchuria's resources.
Debates and open questions
Historians debate how far economics, versus ideology and particular leaders' choices, explains the war; other hard-hit democracies did not turn aggressive.
▸Fascism and Nazi Germany (Mussolini, Hitler) launched World War IIWell documented
The connection
Nazi ideology demanded 'living space' (Lebensraum) in the East and the overturning of Versailles. After taking Austria and Czechoslovakia, Germany invaded Poland on September 1, 1939, having secretly agreed with Stalin to divide it. Britain and France declared war. Mussolini's Italy joined in June 1940, and Japan allied with both.
When and how
Central Europe, 1938–1940; the Nazi-Soviet Pact was signed on August 23, 1939, and the Tripartite Pact linked Germany, Italy and Japan in September 1940.
How we know
Captured German documents, such as the Hossbach Memorandum recording a 1937 planning meeting, and the Nuremberg trials' evidence on planning aggressive war.
Debates and open questions
Historians long debated whether Hitler followed a fixed plan for war or seized chances as they came; most now see clear aims pursued opportunistically.
▸The Holocaust took place during World War IIWell documented
The connection
The war made the Holocaust possible on a continental scale. Conquest brought millions of Jews in Poland, the Soviet Union and elsewhere under Nazi control, and the 1941 invasion of the USSR began the mass shootings. War gave cover and secrecy. Allied victory ended it, as soldiers liberated the camps in 1944–45.
When and how
Occupied Poland, the Soviet Union and the rest of Nazi-ruled Europe, 1939–1945; Soviet troops reached Auschwitz on January 27, 1945, now International Holocaust Remembrance Day.
How we know
German records tie killing operations to military campaigns, and Allied liberation reports and films documented the camps.
Debates and open questions
Historians debate whether the Allies could have done more, such as bombing the rail lines to Auschwitz, and how the war's course shaped the timing of Nazi decisions.
▸Battle of Stalingrad turned the tide of World War IIWell documented
The connection
Stalingrad ended Germany's string of victories in the east and destroyed an entire German army. Coming soon after British victory at El Alamein and US victory at Midway, it marked the shift of momentum to the Allies. Germany never again won a major strategic offensive on the Eastern Front.
When and how
Southern Russia, winter 1942–43; Germany announced the defeat on February 3, 1943, and declared three days of national mourning.
How we know
German and Soviet records, and Nazi propaganda's own shift: Goebbels gave his 'total war' speech on February 18, 1943, weeks after the surrender.
Debates and open questions
Some historians argue Germany could never have won in the east, making Stalingrad less a turning point than a confirmation.
▸D-Day landings in Normandy opened a second front in World War IIWell documented
The connection
Stalin had long demanded a western front to relieve the Soviets, who faced most of Germany's army. D-Day put Allied armies back into western Europe after four years. Germany now had to fight on two major fronts plus Italy, and within 11 months, with Soviet forces taking Berlin, it surrendered.
When and how
Normandy, June 6, 1944, then across France, Belgium and the Netherlands into Germany; Eisenhower was supreme commander, and Montgomery led the ground forces at first.
How we know
Allied and German military records show Germany shifting divisions west; the 1943 Tehran Conference records the Allied commitment to the invasion.
Debates and open questions
Whether the invasion could have come in 1943, as Stalin wanted, and whether the delay was military caution or strategy, is debated.
▸Atomic bombings of Hiroshima and Nagasaki helped end World War IIDebated
The connection
Japan surrendered within days of the bombings, and Emperor Hirohito's August 15 broadcast cited a 'new and most cruel bomb'. But the Soviet Union also declared war on August 8 and invaded Manchuria, ending Japan's hope of Soviet mediation. Historians disagree on which shock mattered more, and whether surrender would have come without the bombs.
When and how
Tokyo's Supreme War Council, August 9–14, 1945; the emperor broke a deadlock in favor of surrender, despite an attempted coup by army officers.
How sure are we?
Japanese leaders' records and postwar interviews, US intercepts, and the emperor's broadcast support both the bombs and the Soviet entry as factors.
Debates and open questions
Tsuyoshi Hasegawa argues the Soviet entry was decisive; Richard Frank stresses the bombs. Whether an invasion would otherwise have been needed is unresolved.
▸World War II led to the United NationsWell documented
The connection
The Allies called themselves the 'United Nations' from 1942. Determined to fix the League of Nations' failures, they planned a stronger body during the war, with great-power vetoes to keep the US and USSR inside. Fifty nations signed the Charter at San Francisco in June 1945, before the Pacific war had ended.
When and how
Declaration by United Nations (January 1, 1942), Dumbarton Oaks talks (1944), Yalta (1945) and the San Francisco Conference (April–June 1945); Roosevelt coined the name.
How we know
The UN Charter, conference records, and Roosevelt's and Churchill's correspondence document the wartime planning.
Debates and open questions
Whether the veto was a necessary compromise or a flaw that paralyzed the Security Council during the Cold War is still argued.
▸World War II gave way to the Cold WarWell documented
The connection
With Germany and Japan defeated and Britain and France weakened, only the US and USSR remained as great powers, with opposing systems. Disputes over Poland, Germany and Eastern Europe soon broke their alliance. In 1947 the US announced the Truman Doctrine to contain communism, and Europe split into rival blocs.
When and how
Yalta and Potsdam (1945), Korea's division at the 38th parallel (1945), Churchill's 'Iron Curtain' speech in Missouri (1946), the Truman Doctrine (1947) and the Berlin Blockade (1948–49).
How we know
Conference records, diplomatic cables such as George Kennan's 1946 'Long Telegram', and Soviet archives opened after 1991.
Debates and open questions
Historians debate whether the Cold War was inevitable once the war ended, or whether different choices by Truman or Stalin might have avoided it.
▸World War II exhausted the empires, speeding Decolonization: the end of empiresMostly agreed
The connection
The war bankrupted Britain and humbled France and the Netherlands, both occupied by Germany. Japan's conquest of Southeast Asia shattered the myth of European invincibility. Millions of colonial soldiers fought for the Allies, whose 1941 Atlantic Charter promised self-determination. Afterward, the colonial powers lacked the money and will to hold on.
When and how
Asia first: India and Pakistan (1947), Burma and Ceylon (1948), Indonesia (1949) after war with the Dutch; Vietnam's war against France ended at Dien Bien Phu in 1954.
How sure are we?
Britain's postwar debts and US loan records, colonial office files, and nationalist leaders' speeches citing wartime promises and Japan's early victories.
Debates and open questions
Historians debate how much the war itself mattered, versus nationalist movements that were already gaining strength before 1939.
▸Attack on Pearl Harbor brought the United States into World War IIWell documented
The connection
The US had been aiding Britain and China but stayed out of the fighting. The attack united American opinion: Congress declared war on Japan on December 8, 1941, with one dissenting vote. Germany and Italy declared war on the US on December 11, joining the European and Pacific wars into one global conflict.
When and how
Washington, December 8–11, 1941; Roosevelt called December 7 'a date which will live in infamy'.
How we know
Congressional records, Roosevelt's speech, and Germany's formal declaration of war on December 11, 1941.
Debates and open questions
Why Hitler declared war on the US, which his treaty with Japan did not require, is still debated.