| 21 August 1056 | Byzantine Empire [political events] | The Byzantine empress Theodora dies. She is the last of the Macedonian dynasty. She is succeeded by her designated heir Michael VI the Aged, a retired civil servant. |
| 21 August 1131 | Kingdom of Jerusalem [administration] | King Baldwin II of Jerusalem dies; he is succeeded by his son-in-law Fulk V le juene of Anjou. |
| 21 August 1157 | Castile, Spain, León [administration] | King Alfonso VII of Castile and León, Emperor of Spain, dies; by his will his sons, Sancho III and Ferdinand II, become kings of Castile and León respectively. |
| 21 August 1165 | France [births and deaths] | Philip II (Philip Augustus), King of France 1179–1223, who reconquered French territories lost previously to England, born in Paris (–1223). |
| 21 August 1271 | Toulouse, France [political events] | Alfonse of Poitiers dies. His nephew, King Philip III of France, thus inherits the counties of Poitou and Toulouse, which are absorbed into the royal domain. Philip presents the Comtât Venaissin, east of the River Rhône, to the papacy. |
| 21 August 1940 | Russia [births and deaths] | Leon Trotsky, communist theorist and activist, a leader in Russia's October Revolution of 1917, is assassinated in Coyaocán, near Mexico City, Mexico (61). |
| 21 August–7 October 1944 | USA, China, USSR, UK [diplomacy] | At the Dumbarton Oaks Conference in Washington, DC, China, the USSR, the USA, and Britain draw up proposals for a new world organization, the future United Nations (UN). |
| 21 August 1944 | France, Germany [World War II (1939–45)] | Allied troops encircle a large concentration of German armoured forces to the south of the town of Falaise, northwest France, as US forces link up with the steady British and Canadian advance southwards. Some 50,000 Germans are captured. |
| 21 August 1959 | Turkey [international organizations] | Following the withdrawal of Iraq from the Baghdad Pact on 24 March, the Pact changes its name to the Central Treaty Organization (CENTO) and moves its headquarters to Ankara, Turkey. |
| 21 August 1983 | Philippines [political events] | The Philippines opposition leader Benigno Aquino is assassinated at Manila airport. |
| 21 August 1991 | USSR [political events] | The coup in the USSR against President Mikhail Gorbachev fails; Gorbachev returns to Moscow the following day. |
| 21 August 1991 | Latvia, USSR [political events] | The Soviet republic of Latvia declares its independence from the USSR. |
