The deadliest conflict in history: from Poland to Pearl Harbor, Stalingrad to Hiroshima, and the world it remade.
Last updated: October 2026
World War II (1939-1945) was the largest and deadliest conflict in human history. It was fought on every continent except Antarctica, on land, at sea, and in the air, and it ended with the defeat of Germany, Italy, and Japan, the founding of the United Nations, and the beginning of the nuclear age.
The war grew out of the unresolved grievances of World War I, the global economic crisis of the Great Depression, and the rise of aggressive dictatorships in Germany, Italy, and Japan. It combined conventional battles between armies, fleets, and air forces with a war against civilians on an unprecedented scale: the bombing of cities, the siege of Leningrad, the Holocaust, and the atomic bombings of Hiroshima and Nagasaki.
Estimates of the total death toll vary widely by source, but most historians place it in the range of fifty to eighty million people, with around seventy million the figure most often cited. The Soviet Union and China suffered the greatest losses. The war destroyed cities across Europe and Asia, displaced tens of millions of people, and redrew the map of the world, leaving the United States and the Soviet Union as the two dominant superpowers and setting the stage for the Cold War.
The origins of World War II lie in the peace settlement of 1919, in the economic catastrophe of the 1930s, and in the ambitions of three expansionist powers. The Treaty of Versailles, which ended World War I, left Germany resentful and politically unstable. The treaty stripped Germany of territory, limited its armed forces, and required reparations. In Germany, politicians across the spectrum blamed the treaty for the country's humiliation, and the myth that the German army had been stabbed in the back in 1918, rather than defeated, poisoned the politics of the Weimar Republic.
The Great Depression, which began with the American stock market crash of 1929, deepened the crisis. Unemployment soared across the industrial world, trade collapsed, and democratic governments seemed unable to respond. In Germany, mass unemployment discredited the Weimar Republic and drove voters toward extremes. In Japan, the Depression strengthened the military's argument that Japan needed an empire in Asia to secure food and raw materials. In the United States and Britain, the Depression encouraged a turn inward, away from international commitments.
In Germany, the National Socialist Party under Adolf Hitler came to power in January 1933. Hitler's programme combined extreme nationalism, racial ideology, and a demand to overturn Versailles and win living space in Eastern Europe. Once in power, he moved quickly: he withdrew Germany from the League of Nations, rebuilt the armed forces in defiance of Versailles, remilitarised the Rhineland in 1936, annexed Austria in March 1938, and demanded the Sudetenland from Czechoslovakia. At Munich in September 1938, Britain and France agreed to the transfer of the Sudetenland in the hope of avoiding war, a policy later known as appeasement. Six months later, Germany occupied the rest of Czechoslovakia, showing that Hitler's ambitions went far beyond reuniting German-speaking peoples.
In Italy, Benito Mussolini had ruled as dictator since 1922. His fascist regime glorified war and empire. Italy invaded Ethiopia in 1935, defying the League of Nations, and intervened in the Spanish Civil War from 1936 to 1939 on the side of Francisco Franco. In 1939, Italy invaded Albania. In Japan, military leaders drove expansion into China, invading Manchuria in 1931 and launching a full-scale war against China in 1937. The war in China, including the mass killing of civilians in Nanjing in late 1937, foreshadowed the brutality of the wider conflict.
The failure of collective security was a further cause. The League of Nations, created after World War I to prevent war, proved unable to stop aggression in Manchuria, Ethiopia, or Spain. Britain and France, the League's leading members, were reluctant to risk war, weakened by the Depression, and haunted by the memory of the trenches. The United States, which had never joined the League, passed neutrality laws designed to keep it out of foreign wars. The Soviet Union, isolated and suspicious of the West, signed a non-aggression pact with Germany in August 1939, the Molotov-Ribbentrop Pact, which included a secret protocol dividing Poland and Eastern Europe into German and Soviet spheres of influence.
The immediate trigger was Germany's invasion of Poland on 1 September 1939. Britain and France, which had guaranteed Poland's independence, declared war on Germany on 3 September. The Soviet Union invaded eastern Poland on 17 September. Poland was defeated within weeks. The war that followed would last nearly six years.
The war can be divided into three broad phases: Axis expansion from 1939 to late 1941, a turning point from 1942 to 1943, and the Allied advance to victory from 1943 to 1945. In each phase, the fighting took place across several theatres at once, and events in one theatre shaped the others.
The first phase was a series of rapid German victories. In the spring of 1940, Germany invaded Denmark and Norway, then struck west. Through the Ardennes forest, German armoured divisions bypassed the French fortifications of the Maginot Line, trapped the British and French armies in Belgium, and drove them to the sea. The evacuation from Dunkirk in May and June 1940 rescued more than 300,000 British and French troops, but France fell, and an armistice was signed on 22 June 1940. Britain stood alone. The Battle of Britain, fought in the skies over southern England in the summer and autumn of 1940, ended Germany's hope of a quick invasion, as the Royal Air Force defeated the German Luftwaffe. The Blitz, the night bombing of London and other British cities, failed to break British morale.
In the Mediterranean and North Africa, Italian and later German forces under Erwin Rommel fought British and Commonwealth forces in Libya and Egypt. In the Balkans, Germany invaded Yugoslavia and Greece in April 1941 to rescue a stalled Italian campaign. In the east, the war expanded dramatically on 22 June 1941, when Germany invaded the Soviet Union in Operation Barbarossa. Three million German troops, supported by allies from Finland, Romania, Hungary, and Italy, drove toward Leningrad, Moscow, and Ukraine. The invasion was the largest military operation in history and it was fought with extreme brutality. The German plan assumed a quick victory, but Soviet resistance, vast distances, and the Russian winter halted the advance short of Moscow by December 1941.
The war became truly global on 7 December 1941, when Japan attacked the American naval base at Pearl Harbor in Hawaii, killing more than 2,400 Americans. The United States declared war on Japan the next day, and Germany and Italy declared war on the United States on 11 December. In the months that followed, Japan conquered a vast empire across Southeast Asia and the western Pacific, including the Philippines, Malaya, Singapore, the Dutch East Indies, and Burma. The fall of Singapore in February 1942, when some 80,000 British and Commonwealth troops surrendered, was one of the worst defeats in British military history.
The turning point came in 1942 and 1943. In the Pacific, the United States Navy won the Battle of Midway in June 1942, sinking four Japanese aircraft carriers and halting Japanese expansion. American forces then began an island-hopping campaign across the Pacific, taking Guadalcanal, Tarawa, Saipan, Iwo Jima, and Okinawa in battles of extraordinary ferocity. In North Africa, the British victory at El Alamein in October and November 1942, followed by the Anglo-American landings in North Africa, drove Axis forces out of Africa by May 1943. In the Atlantic, Allied convoy escorts, radar, codebreaking, and long-range aircraft gradually defeated the German U-boat campaign that had threatened Britain's supply lines.
On the Eastern Front, the Battle of Stalingrad, from August 1942 to February 1943, was the decisive battle of the war in Europe. The German Sixth Army was encircled and destroyed, at a cost of hundreds of thousands of dead on each side, and the initiative on the Eastern Front passed to the Soviet Union. The Battle of Kursk in July 1943, the largest tank battle in history, ended Germany's last major offensive in the east. From then on, the Red Army advanced steadily westward, liberating Ukraine, Belarus, Poland, and the Balkans, at an immense cost in lives.
In the west, the Allies invaded Sicily in July 1943 and mainland Italy in September 1943. Mussolini was overthrown, and Italy surrendered, but German forces occupied northern Italy and fought a slow defensive campaign up the peninsula that lasted until 1945. The decisive western operation was D-Day, 6 June 1944, when American, British, Canadian, and other Allied forces landed on the beaches of Normandy in the largest amphibious invasion in history. Paris was liberated in August 1944. After the German counteroffensive in the Ardennes, the Battle of the Bulge, in December 1944, the Allies crossed the Rhine in March 1945 and advanced into the heart of Germany.
The war in Asia continued after the fall of Germany. American bombing, including the firebombing of Tokyo in March 1945, devastated Japanese cities. The Battle of Okinawa, from April to June 1945, cost tens of thousands of American and Japanese lives, and a far larger number of Okinawan civilian deaths. On 6 August 1945, the United States dropped an atomic bomb on Hiroshima, and on 9 August a second bomb on Nagasaki. The Soviet Union declared war on Japan on 8 August and invaded Manchuria. Japan surrendered on 15 August, and the formal surrender was signed aboard the battleship USS Missouri on 2 September 1945. The war was over.
Leader of the National Socialist Party and dictator of Germany from 1933 until his death. Hitler rearmed Germany, annexed Austria and Czechoslovakia, ordered the invasion of Poland that began the war, and directed the invasion of the Soviet Union and the Holocaust. His racial ideology, which held that Germans were a master race and that Jews, Slavs, Roma, and others were enemies to be eliminated, drove the regime's crimes. He died by suicide in his Berlin bunker on 30 April 1945 as Soviet troops closed in.
British prime minister from May 1940 to July 1945 and again from 1951 to 1955. Churchill became prime minister as France fell and Britain stood alone. His speeches in 1940, including his promise of blood, toil, tears, and sweat, rallied British resistance during the Battle of Britain and the Blitz. He forged the alliance with the United States and the Soviet Union and pressed for the invasion of Normandy, though he often clashed with his allies over strategy.
Leader of the Soviet Union from the late 1920s until his death. Stalin signed the 1939 non-aggression pact with Hitler, then led the Soviet war effort after Germany's invasion in 1941. Under his command, the Red Army absorbed enormous losses and eventually drove the Germans back to Berlin. Stalin also presided over purges, forced collectivisation, and the Gulag camps, which killed millions of Soviet citizens before, during, and after the war.
President of the United States from 1933 until his death in April 1945. Roosevelt led the United States through the Great Depression and, after Pearl Harbor, through most of the war. Before American entry, he supported Britain and the Soviet Union through the Lend-Lease programme. He met Churchill and Stalin at Tehran in 1943 and Yalta in 1945 to plan the post-war order, but he died weeks before the German surrender and was succeeded by Harry Truman.
Fascist dictator of Italy from 1922 until his fall in 1943. Mussolini created the first fascist regime in Europe and inspired Hitler's movement, but his military ambitions outran Italy's strength. Italy's invasions of Ethiopia, Albania, and Greece, and its campaigns in North Africa, repeatedly required German rescue. After the Allied invasion of Sicily, the Italian king dismissed Mussolini. He was later captured and executed by Italian partisans in April 1945.
Japanese general and prime minister from October 1941 to July 1944. Tojo presided over the decision to attack Pearl Harbor and the expansion of the war across Asia and the Pacific. After Japan's defeat, he was tried by the International Military Tribunal for the Far East in Tokyo, convicted of war crimes, and executed in 1948.
French general who, after the fall of France in June 1940, refused the armistice and led the Free French from London. His broadcast of 18 June 1940 called on the French to continue the fight. De Gaulle built the Free French forces into a significant army, led the provisional government that took power after the liberation of Paris, and later founded the Fifth Republic, serving as its first president from 1959 to 1969.
American general who served as supreme commander of Allied forces in Europe. Eisenhower commanded the invasion of North Africa in 1942, the invasion of Sicily and Italy in 1943, and the D-Day landings in Normandy in 1944. His skill at holding together a coalition of American, British, and other commanders was central to the Allied victory in the west. He later served as the 34th president of the United States from 1953 to 1961.
Soviet marshal and the most important Soviet commander of the war. Zhukov directed the defence of Moscow in 1941, coordinated the encirclement of the German army at Stalingrad, led the offensive at Kursk, and commanded the final assault on Berlin. His career reflected both the strengths and the brutality of the Soviet war effort, in which victory was pursued regardless of cost.
German field marshal known as the Desert Fox for his command of the Afrika Korps in North Africa from 1941 to 1943. Rommel won a series of victories against British forces before being defeated at El Alamein. He later commanded German defences in Normandy. In 1944, after being implicated in the plot to assassinate Hitler, he was forced to take his own life.
A Jewish girl from Amsterdam whose diary, written while her family hid from the Nazis, became one of the most widely read books of the twentieth century. The Frank family was betrayed in August 1944 and deported. Anne died in the Bergen-Belsen concentration camp in early 1945, shortly before its liberation. Her diary remains a central document of the Holocaust for readers worldwide.
The human cost of World War II defies full comprehension. The Soviet Union lost an estimated 27 million people, military and civilian. China lost an estimated 15 to 20 million. Poland lost about six million, including about three million Polish Jews. Germany lost several million soldiers and civilians. Japan lost about three million. The United States and Britain, by comparison, lost hundreds of thousands each. These figures are estimates, and historians continue to revise them, but the scale of the catastrophe is not in doubt.
The Holocaust was the war's defining crime. The Nazi regime systematically murdered about six million Jews, along with Roma, disabled people, Soviet prisoners of war, Polish civilians, political opponents, and others. The Nuremberg trials after the war, at which leading Nazis were tried for war crimes, crimes against humanity, and crimes against peace, established the principle that individuals, including heads of state, could be held criminally responsible for such acts under international law.
The political map of the world was transformed. Germany was divided into occupation zones, and Berlin was divided as well. The division hardened into two German states: the Federal Republic of Germany in the west and the German Democratic Republic in the east. Poland's borders moved westward. The Soviet Union annexed the Baltic states and territory from Finland, Romania, and Poland, and installed communist governments across Eastern Europe. In Asia, Korea was divided, setting the stage for the Korean War, and the Chinese Civil War resumed, ending in communist victory in 1949.
The United Nations was founded in 1945 to replace the League of Nations, with a Security Council, a General Assembly, and a broader membership. The Bretton Woods institutions, the International Monetary Fund and the World Bank, were created to stabilise the international economy. The Universal Declaration of Human Rights, adopted in 1948, set out rights intended to apply to all people, a direct response to the war's atrocities.
The war also launched the nuclear age and the Cold War. The atomic bombings of Hiroshima and Nagasaki demonstrated a weapon capable of destroying a city in a single blast. The wartime alliance between the United States, Britain, and the Soviet Union broke down within a few years, as the Soviet Union consolidated control over Eastern Europe and the West formed the NATO alliance in 1949. The Cold War that followed would dominate world politics for the next four decades.
Decolonisation accelerated. The war weakened the European colonial powers, and independence movements in India, Indonesia, Vietnam, Algeria, and elsewhere drew strength from the war's rhetoric of freedom and from the visible weakness of the colonial powers. India gained independence in 1947. Indonesia declared independence from the Netherlands in 1945. The process continued for decades and reshaped the membership of the United Nations.
The war left a lasting legacy in memory and law. Holocaust remembrance, war crimes tribunals, and the Geneva Conventions of 1949, which strengthened protections for civilians, prisoners of war, and the wounded, all grew from the experience of the war. The war also left an enormous cultural legacy, in films, novels, memoirs, and museums, and it remains the central reference point for debates about aggression, appeasement, genocide, and the responsibilities of democratic states in the face of dictatorship.
