| 12 September 1185 | Byzantine Empire, Normandy [wars] | The Byzantine emperor Andronicus I Comnenus is killed in a riot in Constantinople; he is succeeded by his cousin, Isaac II Angelus, who defeats the Normans at Mosinopolis and expels them. |
| 12 September 1213 | France, Toulouse [Crusades (1095–1272)] | The Norman crusader Simon IV de Montfort and the Albigensian crusaders defeat Raymond VI, Count of Toulouse, and kill his ally King Pedro II of Aragon at Muret. Pedro's heir in Aragon is his young son, James I, and civil war follows there. |
| 12 September 1217 | England, France [treaties] | By the Treaty of Kingston, the English rebels make peace with King Henry III of England and the French dauphin Louis is paid to leave England. |
| 12 September 1303 | Italy [political events] | Pope Boniface VIII, imprisoned in Anagni, Italy, by an Italian force for King Philip IV of France, is released in a rising by the people of Anagni. |
| 12 September 1683 | Poland, Austria, Germany, Ottoman Empire, Hungary, Habsburg Monarchy, Holy Roman Empire [political events] | Allied Austro-Polish forces under King John III Sobieski and Duke Charles of Lorraine, assisted by forces from Bavaria and Saxony, raise the Ottoman siege of Vienna. The Grand Vizier Kara Mustafa is made a scapegoat for the defeat and is executed on 25 December. The victory is a preliminary to the Habsburg reconquest of Hungary. |
| 12 September 1683 | Portugal [administration] | Pedro II accedes to the throne of Portugal on the death of his brother King Afonso VI. He has acted as Prince Regent since 1668. |
| 12 September 1801 | Russian Empire [political events] | Tsar Alexander I of Russia announces the annexation of the kingdom of Georgia and George XIII, Regent of Georgia, recognizes the Russian decision instead of accepting the traditional suzerainty of Persia. |
| 12 September 1848 | Switzerland [law and government] | Following the defeat of the Sonderbund, the armed league of the seven Catholic cantons, Switzerland adopts a new constitution by which the states become a federal union with strong central government. |
| 12 September 1919 | Italy, Kingdom of the Serbs Croats and Slovenes [wars] | The poet and nationalist Gabriele d'Annunzio leads an unofficial Italian army to seize the northern Adriatic port of Fiume before it is incorporated into the Kingdom of the Serbs, Croats, and Slovenes. |
| 12 September 1922 | USA [Protestantism] | The US Protestant Episcopal Church changes its marriage ceremony, deleting the word ‘obey’ from the vows. |
| 12 September 1934 | Estonia, Latvia, Lithuania [treaties] | The Baltic states (Estonia, Latvia, and Lithuania) sign a Treaty of Understanding and Cooperation (known as the ‘Baltic Entente’) in Geneva, Switzerland. |
| 12 September 1935 | USA [aircraft] | US multimillionaire Howard Hughes sets the world's airspeed record of 567.23 kph/352.46 mph, in an aeroplane of his own design. |
| 12 September 1955 | England [transport] | English engineer Christopher Cockerell patents the first hovercraft. |
| 12 September 1958 | USA [information technology] | US electrical engineer Jack Kilby demonstrates the first integrated circuit. It consists of transistors, resistors, and capacitors contained within a silicon substrate. It leads to the third generation of computers. |
| 12 September 1968 | Albania [international organizations] | Albania formally quits the Warsaw Pact. |
| 12 September 1974 | Ethiopia [political events] | A military coup in Ethiopia deposes Emperor Haile Selassie. |
| 12 September 1989 | Poland [political events] | A Solidarity-dominated cabinet is formed in Poland under Tadeusz Mazowiecki, the first noncommunist government in Eastern Europe since 1948. |
