August 21 is the 233rd day of the year (234th in leap years) in the Gregorian calendar; 132 days remain until the end of the year.
1169 – Battle of the Blacks: Uprising by the black African forces of the Fatimid army, along with a number of Egyptian emirs and commoners, against Saladin.
1192 – Minamoto no Yoritomo becomes Sei-i Taishōgun and the de facto ruler of Japan.
1331 – King Stefan Uroš III, after months of anarchy, surrenders to his son and rival Stefan Dušan, who succeeds as King of Serbia 1680 – Pueblo Indians capture Santa Fe from the Spanish during the Pueblo Revolt
1689 – The Battle of Dunkeld in Scotland
1770 – James Cook formally claims eastern Australia for Great Britain, naming it New South Wales.
1772 – King Gustav III completes his coup d'état by adopting a new Constitution, ending half a century of parliamentary rule in Sweden and installing himself as an enlightened despot.
1778 – American Revolutionary War: British forces begin besieging the French outpost at Pondichéry.
1791 – A Vodou ceremony, led by Dutty Boukman, turns into a violent slave rebellion, beginning the Haitian Revolution.
1810 – Jean-Baptiste Bernadotte, Marshal of France, is elected Crown Prince of Sweden by the Swedish Riksdag of the Estates.
1831 – Nat Turner leads black slaves and free blacks in a rebellion in Southampton County, Virginia, which will claim the lives of 55 to 65 whites and about twice that number of blacks
1852 – Tlingit Indians destroy Fort Selkirk, Yukon Territory.
1863 – Lawrence, Kansas is destroyed by pro-Confederate guerrillas known as Quantrill's Raiders.
1878 – The American Bar Association is founded in Saratoga Springs, New York
1879 – The locals of Knock, County Mayo, Ireland report their having seen an apparition of the Virgin Mary. The apparition is later named “Our Lady of Knock” and the spot transformed into a Catholic pilgrimage site.
1883 – An F5 tornado strikes Rochester, Minnesota, leading to the creation of the Mayo Clinic.
1888 – The first successful adding machine in the United States is patented by William Seward Burroughs.
1911 – The Mona Lisa is stolen by Vincenzo Peruggia, a Louvre employee
1914 – World War I: The Battle of Charleroi, a successful German attack across the River Sambre that pre-empted a French offensive.
1918 – World War I: The Second Battle of the Somme begins.