The Kragujevac massacre was the mass murder of between 2,778 and 2,794 mostly Serb men and boys in Kragujevac by German soldiers on 21 October 1941. It occurred in the German-occupied territory of Serbia during World War II, and came as a reprisal for insurgent attacks in the Gornji Milanovac district that resulted in the deaths of ten German soldiers and the wounding of 26 others. The number of hostages to be shot was calculated as a ratio of 100 hostages executed for every German soldier killed and 50 hostages executed for every German soldier wounded, a formula devised by Adolf Hitler with the intent of suppressing anti-Nazi resistance in Eastern Europe.

After a punitive operation was conducted in the surrounding villages, during which over 400 males were shot and four villages burned down, another 70 male Jews and communists who had been arrested in Kragujevac were killed. Simultaneously, males between the ages of 16 and 60, including high school students, were assembled by German troops and local collaborators, and the victims were selected from amongst them. The selected males were then marched to fields outside the city, shot with heavy machine guns, and their bodies buried in mass graves. Contemporary German military records indicate that 2,300 hostages were shot. After the war, inflated estimates ranged as high as 7,000 deaths, but German and Serbian scholars have now agreed on the figure of nearly 2,800 killed, including 144 high school students. As well as Serbs, massacre victims included Jews, Romani people, Muslims, Macedonians, Slovenes, and members of other nationalities.

Several senior German military officials were tried and convicted for their involvement in the reprisal shootings at the Nuremberg trials and the subsequent Nuremberg trials. The massacre had a profound effect on the course of the war in Yugoslavia. It exacerbated tensions between the two guerrilla movements, the communist-led Partisans and the royalist, Serbian nationalist Chetniks, and convinced Chetnik leader Draa Mihailovi that further attacks against the Germans would only result in more Serb civilian deaths. The Germans soon found mass executions of Serbs to be ineffectual and counterproductive, as they tended to drive the population into the arms of insurgents. The ratio of 100 executions for one soldier killed and 50 executions for one soldier wounded was reduced by half in February 1943, and removed altogether later in the year. The massacre is commemorated by the October in Kragujevac Memorial Park and the co-located 21 October Museum, and has been the subject of several poems and feature films. The anniversary of the massacre is commemorated annually in Serbia as the Day of Remembrance of the Serbian Victims of World War II.

World War II or the Second World War, often abbreviated as WWII or WW2, was a global war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposing military alliances: the Allies and the Axis powers. In a total war directly involving more than 100 million personnel from more than 30 countries, the major participants threw their entire economic, industrial, and scientific capabilities behind the war effort, blurring the distinction between civilian and military resources. Aircraft played a major role in the conflict, enabling the strategic bombing of population centres and the only two uses of nuclear weapons in war. World War II was by far the deadliest conflict in human history; it resulted in 70 to 85 million fatalities, a majority being civilians. Tens of millions of people died due to genocides (including the Holocaust), starvation, massacres, and disease. In the wake of the Axis defeat, Germany and Japan were occupied, and war crimes tribunals were conducted against German and Japanese leaders.

The exact causes of World War II are debated, but contributing factors included the Second Italo-Ethiopian War, the Spanish Civil War, the Second Sino-Japanese War, the Soviet–Japanese border conflicts and rising European tensions since World War I. World War II is generally considered to have begun on 1 September 1939, when Nazi Germany, under Adolf Hitler, invaded Poland. The United Kingdom and France subsequently declared war on Germany on 3 September. Under the Molotov–Ribbentrop Pact of August 1939, Germany and the Soviet Union had partitioned Poland and marked out their "spheres of influence" across Finland, Estonia, Latvia, Lithuania and Romania. From late 1939 to early 1941, in a series of campaigns and treaties, Germany conquered or controlled much of continental Europe, and formed the Axis alliance with Italy and Japan (along with other countries later on). Following the onset of campaigns in North Africa and East Africa, and the fall of France in mid-1940, the war continued primarily between the European Axis powers and the British Empire, with war in the Balkans, the aerial Battle of Britain, the Blitz of the UK, and the Battle of the Atlantic. On 22 June 1941, Germany led the European Axis powers in an invasion of the Soviet Union, opening the Eastern Front, the largest land theatre of war in history.

Japan, which aimed to dominate Asia and the Pacific, was at war with the Republic of China by 1937. In December 1941, Japan attacked American and British territories with near-simultaneous offensives against Southeast Asia and the Central Pacific, including an attack on the US fleet at Pearl Harbor which resulted in the United States declaring war against Japan. Therefore the European Axis powers declared war on the United States in solidarity. Japan soon captured much of the western Pacific, but its advances were halted in 1942 after losing the critical Battle of Midway; later, Germany and Italy were defeated in North Africa and at Stalingrad in the Soviet Union. Key setbacks in 1943—including a series of German defeats on the Eastern Front, the Allied invasions of Sicily and the Italian mainland, and Allied offensives in the Pacific—cost the Axis powers their initiative and forced it into strategic retreat on all fronts. In 1944, the Western Allies invaded German-occupied France, while the Soviet Union regained its territorial losses and turned towards Germany and its allies. During 1944 and 1945, Japan suffered reversals in mainland Asia, while the Allies crippled the Japanese Navy and captured key western Pacific islands.

The war in Europe concluded with the liberation of German-occupied territories, and the invasion of Germany by the Western Allies and the Soviet Union, culminating in the fall of Berlin to Soviet troops, Hitler's suicide and the German unconditional surrender on 8 May 1945. Following the Potsdam Declaration by the Allies on 26 July 1945 and the refusal of Japan to surrender on its terms, the United States dropped the first atomic bombs on the Japanese cities of Hiroshima, on 6 August, and Nagasaki, on 9 August. Faced with an imminent invasion of the Japanese archipelago, the possibility of additional atomic bombings, and the Soviet’s declared entry into the war against Japan on the eve of invading Manchuria, Japan announced on 15 August its intention to surrender, then signed the surrender document on 2 September 1945, cementing total victory in Asia for the Allies.

World War II changed the political alignment and social structure of the globe. The United Nations (UN) was established to foster international co-operation and prevent future conflicts, with the victorious great powers—China, France, the Soviet Union, the United Kingdom, and the United States—becoming the permanent members of its Security Council. The Soviet Union and the United States emerged as rival superpowers, setting the stage for the nearly half-century-long Cold War. In the wake of European devastation, the influence of its great powers waned, triggering the decolonisation of Africa and Asia. Most countries whose industries had been damaged moved towards economic recovery and expansion. Political and economic integration, especially in Europe, began as an effort to forestall future hostilities, end pre-war enmities and forge a sense of common identity.