World War II: The Kragujevac massacre against Serbian men and boys takes place.

World War II or the Second World War was a global conflict between two coalitions: the Allies and the Axis powers. Nearly all of the world's countries participated, with many nations mobilising all resources in pursuit of total war. Tanks and aircraft played major roles, enabling the strategic bombing of cities and delivery of the first and only nuclear weapons ever used in war. World War II is the deadliest conflict in history, causing the death of 70 to 85 million people, more than half of whom were civilians. Millions died in genocides, including the Holocaust, and by massacres, starvation, and disease. After the Allied victory, Germany, Austria, Japan, and Korea were occupied, and German and Japanese leaders were tried for war crimes.

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.

October 21 is the 294th day of the year in the Gregorian calendar; 71 days remain until the end of the year.