| 21 April 1030 | Ghaznavid Emirate, Afghanistan, Persia [births and deaths] | Mahmud III, emir of Ghazni 998–1030, an area covering modern Afghanistan and northeastern Persia, who conquered Kashmir, the Punjab, and much of Persia, also noted as a patron of the arts, dies in Ghazni (c. 59). |
| 21 April 1574 | Florence [births and deaths] | Cosimo I the Great de' Medici, Duke of Florence 1537–74 and Grand Duke of Tuscany 1569–74, dies in Castello, near Florence, Italy (54). |
| 21 April 1699 | France [births and deaths] | Jean Racine, French poet and dramatist known for his tragedies Briannicus, Bérénice, and Phèdre/Phaedra, dies in Paris, France (59). |
| 21 April 1703 | Portugal, UK, United Netherlands, Austria, Habsburg Monarchy, Holy Roman Empire [War of the Spanish Succession (1701–1714)] | Portugal concludes a treaty with Britain and joins the Grand Alliance (Britain, the United Netherlands, and Austria) against France. |
| 21 April 1816 | England [births and deaths] | Charlotte Brontë, English novelist who writes Jane Eyre (1847), born in Thornton, Yorkshire, England (–1855). |
| 21 April 1836 | Republic of Texas, Mexico [wars] | Republic of Texas general Sam Houston defeats his Mexican counterpart General Antonio Lopes de Santa Anna at the Battle of San Jacinto, thereby securing the independence of Texas and ending the Texas-Mexican War. |
| 21 April 1855 | USA [other structures] | The first bridge across the Mississippi, between Rock Island, Illinois, and Davenport, Iowa, opens to rail traffic. |
| 21 April 1910 | [births and deaths] | Mark Twain (pseudonym of Samuel Langhorne Clemens), US author who created the characters Tom Sawyer and Huckleberry Finn, dies in Redding, Connecticut (74). |
| 21 April 1918 | Denmark [suffrage] | Universal suffrage is granted in Denmark. |
| 21 April 1918 | Germany [World War I (1914–18)] | Manfred, Freiherr von Richthofen (the ‘Red Baron’), German aviator and leading ace during World War I, is shot down and killed in Vaux-sur-Somme, France (26). It is not certain whether he was shot down by Canadian pilot Roy Brown or by ground fire. |
| 21 April 1926 | [births and deaths] | Elizabeth II, Queen of Great Britain and Northern Ireland from 1952, born in London, England. |
| 21 April 1941 | Greece, Germany, UK [World War II (1939–45)] | The Greek army surrenders to the invading German forces; the last British forces are evacuated from mainland Greece on 28 April. |
| 21 April 1947 | Eire [shops and shopping] | The first airport duty-free shop is opened at Shannon Airport in Eire. |
| 21 April 1964 | UK [television] | BBC2 begins broadcasting, after a power-cut delays the launch by 24 hours. It broadcasts high-definition pictures, transmitting on 625 lines. |
| 21 April 2000 | Russia [treaties] | The Russian parliament ratifies the comprehensive nuclear test ban treaty. The US Senate refused to ratify the treaty in 1999. |
