| 1941 | April | 21 | With their retreat cut off by the German advance, 223,000 Greek soldiers of the Greek army in Albania surrender. |
| 1944 | April | 21 | The Badoglio government in Italy falls and he is quickly asked to form another. |
| 1944 | April | 21 | An Allied air raid on Paris kills a large number of civilians. |
| 1945 | April | 21 | Soviet forces under Georgiy Zhukov's (1st Belorussian Front), Konstantin Rokossovskiy's (2nd Belorussian Front) and Ivan Konev's (1st Ukrainian Front) launch assaults on the German forces in and around the city of Berlin in the opening stages of the Battle of Berlin. |
| 1945 | April | 21 | Hitler orders SS-General Felix Steiner to attack the 1st Belorussian Front and destroy it. The ragtag units of "Army Detachment Steiner" are not fully manned. |