The roots of World War II lay in the unresolved geopolitical tensions following World War I, the punitive territorial and financial terms of the 1919 peace settlements, and the severe economic destabilization wrought by the Great Depression in the 1930s.⁸ These conditions weakened democratic institutions and fostered the rise of totalitarian, expansionist regimes in Europe and militarism in Asia. In Italy, Benito Mussolini established a Fascist dictatorship that sought a new Mediterranean and African empire, invading Abyssinia (Ethiopia) on October 3, 1935. In Germany, Adolf Hitler and the National Socialist (Nazi) Party took power in January 1933.⁹ By March 1933, the Reichstag passed the Enabling Act, granting Hitler dictatorial authority. He rapidly rearmed Germany, repudiated postwar treaty restrictions, and pursued a racial ideology aimed at territorial expansion in Eastern Europe and the subjugation or extermination of Jews and Slavic peoples.

In East Asia, the Empire of Japan sought control over continental resources to sustain its industrial and military ambitions. While most historians date the start of World War II to September 1, 1939, several scholars point to earlier conflicts in Asia as the opening phases of the global struggle. On September 18, 1931, Japanese forces staged an invasion of Manchuria in northeastern China, establishing the puppet state of Manchukuo.¹⁰¹¹ Tensions escalated into full-scale conflict on July 7, 1937, following the Marco Polo Bridge incident near Beijing, which initiated the Second Sino-Japanese War. Although Japan expected a rapid capitulation, Chinese Nationalist forces under Chiang Kai-shek and Chinese Communist forces mounted protracted resistance, bogging down millions of Japanese troops after a grueling campaign in Shanghai and the capture of Nanjing.

Meanwhile, in Europe, Hitler annexed Austria and the German-speaking Sudetenland region of Czechoslovakia in 1938, testing the Western democracies' policy of appeasement.¹² Germany, Italy, and Japan gradually aligned their interests through strategic agreements that culminated in the formalization of the Axis alliance in 1940.¹³ To prevent a two-front war before moving against Poland, Germany signed a non-aggression pact with the Soviet Union under Joseph Stalin on August 23, 1939, which secretly partitioned Eastern Europe into German and Soviet spheres of influence.¹⁴

World War II in Europe began on September 1, 1939, when German forces invaded Poland using combined-arms tactics centered on fast-moving armored divisions and close air support, a doctrine known as blitzkrieg. After Germany ignored ultimatums to withdraw, the United Kingdom and France declared war on September 3. On September 17, the Soviet Union invaded Poland from the east, and by October 6, Polish organized resistance ended with the country divided between the two powers. The Soviet Union subsequently pressured the Baltic states into allowing Soviet military bases on their soil and, on November 30, 1939, invaded Finland in the Winter War.

Following a quiet winter along the Franco-German border known as the Phoney War, Germany launched an offensive into northern and western Europe in the spring of 1940. On April 9, German forces invaded Denmark and Norway to secure naval bases and iron ore supply routes. On May 10, Germany attacked France and the Low Countries—Belgium, the Netherlands, and Luxembourg—bypassing French border fortifications. British and Allied forces were cut off in northern France, necessitating a mass naval evacuation from Dunkirk beginning on May 26. Italy entered the war on the Axis side on June 10, and France surrendered later that month, resulting in German occupation of the north and west and the establishment of the collaborationist Vichy regime in the south.

With France defeated, the United Kingdom, led by Prime Minister Winston Churchill, stood alone in Western Europe. Beginning on July 10, 1940, the German air force (Luftwaffe) launched the Battle of Britain to achieve air superiority ahead of a planned amphibious invasion. Failing to break the Royal Air Force, Germany shifted on September 7 to a sustained night bombing campaign against British cities. Simultaneously, German submarines waged the Battle of the Atlantic to sever Britain's maritime supply lines from North America, while Axis and British Commonwealth forces clashed across North Africa, the Mediterranean, and the Balkans, where Hungary, Romania, Slovakia, and Bulgaria joined the Axis fold.

On June 22, 1941, Hitler broke the non-aggression pact with Moscow and launched Operation Barbarossa, the invasion of the Soviet Union.¹⁵ More than three million German soldiers, supported by 600,000 troops from Axis partner states and Finland, advanced across a vast front stretching from the Baltic to the Black Sea. Caught unprepared despite intelligence warnings, the Soviet Red Army suffered catastrophic initial losses. Within six months, Axis forces destroyed 124 Soviet divisions, captured 3.8 million prisoners of war, and occupied territory equivalent in size to Britain, France, Germany, and Italy combined. In the north, German and Finnish forces encircled Leningrad in an 872-day siege that caused mass civilian starvation.¹⁶

The German drive stalled at the gates of the Soviet capital during the Battle for Moscow, which lasted from September 1941 to April 1942 and involved roughly seven million combatants, resulting in 2.5 million deaths. In the summer of 1942, Hitler redirected his main offensive south toward the oil fields of the Caucasus and the industrial city of Stalingrad on the Volga River. After months of fierce house-to-house combat, the Red Army launched Operation Uranus, a counteroffensive that encircled the German 6th Army inside the ruined city.¹⁷ Surviving German forces at Stalingrad surrendered on February 2, 1943, marking a decisive psychological and strategic turning point.

Germany attempted one final strategic offensive in the east in July 1943 at the Kursk salient in southwestern Russia. The Battle of Kursk, fought from July 5 to August 23, 1943, became the largest armored battle in history, involving approximately 6,000 tanks and resulting in a decisive Soviet victory. Thereafter, the Red Army held the strategic initiative, liberating Kiev in November 1943 and breaking the Siege of Leningrad in early 1944.¹⁸ The Eastern Front proved the most decisive theater of the European war, accounting for more than 75 to 80 percent of all German military combat casualties and materiel losses.¹⁹

As war raged in Europe, Japan moved to seize European colonial possessions in Southeast Asia to secure vital rubber and oil supplies, occupying French Indochina.²⁰ Facing economic embargoes from the United States, the United Kingdom, and the Netherlands, Japanese leaders decided to neutralize the United States Pacific Fleet before sweeping through the region. Admiral Isoroku Yamamoto, Commander in Chief of the Japanese Combined Fleet, had warned Prime Minister Fumimaro Konoe in 1940 that while Japan could win victories for the first six to twelve months, it could not match American industrial capacity in a prolonged war.²¹

On December 7, 1941, Japanese carrier aircraft executed a surprise attack on the American naval base at Pearl Harbor in Hawaii, alongside coordinated assaults on the Philippines, Malaya, and Hong Kong. The United States and the United Kingdom declared war on Japan the following day, bringing the United States fully into the global conflict. Over the next five months, Japan conquered a maritime and continental empire spanning more than 2.8 million square miles across Southeast Asia and the western Pacific. In the Philippines, U.S. commander General Douglas MacArthur was ordered to evacuate to Australia in March 1942, and in April, approximately 72,000 American and Filipino troops surrendered on the Bataan Peninsula.

Japan's naval expansion was halted in June 1942 at the Battle of Midway, where U.S. Navy carrier forces sank four Japanese fleet carriers, shifting the balance of naval power in the Pacific. Allied forces—primarily from the United States, Australia, New Zealand, Britain, India, and China—Subsequently embarked on a twin-pronged counteroffensive across the Pacific and mainland Asia. Employing an "island-hopping" strategy, American forces bypassed heavily fortified Japanese garrisons to seize strategic airfields and naval bases progressively closer to the Japanese home islands. By late 1944, U.S. forces had landed to retake the Philippines and established bases in the Mariana Islands, placing Japanese cities within range of long-range B-29 strategic bombers.²²

World War II was defined by unprecedented violence against non-combatants. In Europe, Nazi Germany perpetrated the Holocaust, the systematic, state-sponsored genocide of six million European Jews—roughly two-thirds of Europe's Jewish population—alongside the mass murder of millions of ethnic Poles, Soviet prisoners of war, Roma, people with disabilities, homosexuals, and political dissidents. Following the 1941 invasion of the Soviet Union, mobile SS killing squads (Einsatzgruppen) shot over a million Jews and other targeted groups in mass graves across Eastern Europe. The regime subsequently constructed industrialized extermination camps in occupied Poland—including Auschwitz, Treblinka, Belzec, Sobibor, Chełmno, and Majdanek—where victims were murdered in gas chambers. Under its broader Hunger Plan and racial policies, Germany also starved or worked to death over three million Soviet prisoners of war and millones of Slavic civilians.²⁹

In the Asia-Pacific theater, Imperial Japanese forces committed widespread atrocities against civilians and Allied prisoners of war under policies such as the "Three Alls" (kill all, burn all, loot all) in China. During the 1937–1938 Nanjing Massacre, Japanese troops murdered and raped hundreds of thousands of Chinese civilians and disarmed combatants.³⁰ In Harbin, Manchuria, the Imperial Japanese Army operated Unit 731, a covert biological and chemical warfare research complex headed by General Shiro Ishii. Between 1936 and 1945, Japanese military physicians subjected thousands of Chinese, Korean, Soviet, and Mongolian prisoners to lethal vivisections, frostbite tests, and deliberate infection with plague, anthrax, cholera, and typhoid, leaving no known survivors inside the facility.³¹ Declassified Soviet interrogation records from the 1949 Khabarovsk War Crimes Trials and Chinese archival estimates indicate that at least 3,000 to over 14,000 victims perished inside Unit 731's facilities, while Japanese biological weapon attacks in Chinese cities killed more than 300,000 people.³²

Allied powers also engaged in actions that caused massive civilian suffering or violated international norms.³³ In the Soviet Union, Stalin's regime carried out mass deportations, executions of Polish officers and intellectuals, and terror campaigns in annexed territories. Both Axis and Allied air forces conducted strategic bombing campaigns against urban centers—such as the German bombing of London and Warsaw, and the Anglo-American firebombing of German and Japanese cities—that killed hundreds of thousands of civilians. On the American home front, the U.S. government forcibly relocated and interned more than 100,000 Japanese Americans in interior camps for the duration of the conflict.

Estimates of total World War II fatalities vary because wartime records were destroyed or incomplete, ranging from 50 million to 85 million dead—roughly 3 percent of the 1940 global population of 2.3 billion. Approximately 21 million to 25 million military personnel died, alongside 50 million to 55 million civilians who perished from direct violence, genocide, and war-induced famine and disease. The Soviet Union suffered the highest human cost, with official post-Soviet Russian research placing total fatalities at 26.6 million (including 8 to 9 million from famine and disease). China suffered between 15 million and 20 million deaths (with total killed and wounded exceeding 50 million in some estimates). Poland lost between 5.6 million and 5.8 million citizens, representing roughly 20 percent of its prewar population. Military and civilian deaths totaled over 5.3 million to 7 million for Germany and approximately 2 million to 3 million for Japan.

Following the war, the Allies occupied Germany, Austria, Japan, and Korea. To prosecute Axis leaders for crimes against peace, war crimes, and crimes against humanity, the four major Allied powers convened the International Military Tribunal at Nuremberg, Germany, from November 1945 to October 1946.³⁴³⁵ Of 24 indicted senior Nazi officials, 19 were convicted and 12 were sentenced to death; the United States subsequently held 12 additional Nuremberg trials through 1949. In Japan, General MacArthur established the International Military Tribunal for the Far East (the Tokyo Trials, 1946–1948) to prosecute senior Japanese leaders, though U.S. authorities controversially granted immunity to Shiro Ishii and key Unit 731 researchers in exchange for their biological warfare data.³⁶

To prevent future global conflicts, the victorious Allies established the United Nations in 1945, placing the five principal Allied powers—the United States, the Soviet Union, the United Kingdom, France, and China—as permanent members of the UN Security Council. The Nuremberg Principles laid the legal foundation for the 1948 Genocide Convention, the Universal Declaration of Human Rights, and expanded Geneva Conventions. However, wartime cooperation between the Western Allies and the Soviet Union rapidly dissolved into the Cold War. Europe was divided between a U.S.-aligned Western Bloc rebuilt under the Marshall Plan and a Soviet-dominated Eastern Bloc, initiating a forty-five-year ideological standoff and nuclear arms race that defined the second half of the twentieth century.