| 20 August 917 | Bulgar Khanate, Byzantine Empire [wars] | The Bulgar khan, Symeon, demands that he should be recognized as Byzantine emperor, after defeating the imperial forces near Anchialus on the River Achelous, invading Thrace, and making himself master of the Balkans. |
| 20 August 1153 | France [births and deaths] | St Bernard of Clairvaux, influential Cistercian abbot and mystic, founder of the abbey of Clairvaux, dies in Clairvaux, France (c. 63). |
| 20 August 1503 | Venice, Italy, Hungary, Ottoman Empire, Balkans [treaties] | Venice, Hungary, and the Ottoman Empire conclude a peace. Under the treaty Venice is to abandon Lepanto, Modon, Coron, and Navarino in Morea (the Peloponnese, Greece) and Durazzo in Albania, but retain Cephalonia. Hungary agrees a seven-year truce with the Turks. |
| 20 August 1640 | UK [wars] | A Scottish army crosses the River Tweed into England, beginning the Second Bishops' War. The war, between the Scottish Covenanters and Charles I of England, is caused by Charles' unpopular policies against the Scottish Kirk and ended in English defeats and bankruptcy for Charles. |
| 20 August 1648 | France, Austria, Spain, Holy Roman Empire, Habsburg Monarchy [Thirty Years War (1618–48)] | French forces prevent an Austro-Spanish invasion of Paris, France, when they defeat the allied armies at Lens, Artois. The failure of the Holy Roman Emperor Ferdinand III's plans quickens the pace of the peace negotiations in Münster and Osnabrück. |
| 20 August 1672 | United Netherlands [crime and punishment] | Having been held responsible for Dutch failures in the war against France and having resigned as Grand Pensionary earlier in the month, Johan de Witt, political leader of the United Netherlands (1653–72), who led his country during the First and Second Anglo-Dutch Wars, and his brother Cornelius are assassinated by a large mob at The Hague in the United Netherlands (47). |
| 20–21 August 1886 | Ottoman Empire [revolution] | A military coup is effected in Sofia, Bulgaria, by discontented pro-Russian army officers. |
| 20 August 1908 | Belgium [colonies and mandate] | King Leopold II of Belgium transfers the Congo to Belgium, having previously exploited it through a private company. |
| 20 August 1912 | [births and deaths] | William Booth, English preacher who founded the Salvation Army, dies in London, England (83). |
| 20 August 1968 | Czechoslovakia, USSR [wars] | Soviet and other Warsaw Pact forces invade Czechoslovakia and arrest reform leaders including Alexander Dubcek. |
| 20 August 1980 | Asia [mountaineering] | Reinhold Messner of Italy completes the first entirely solo ascent of Mount Everest, the world's highest mountain. |
| 20 August 1991 | Estonia, USSR [political events] | The Soviet republic of Estonia declares its independence from the USSR. |
| 20 August 2001 | England [births and deaths] | Fred(erick) Hoyle, English astronomer, scientist, and science fiction writer, dies in Bournemouth, England (86). |
