October 27 is the 300th day of the year in the Gregorian calendar. There are 65 days remaining until the end of the year.
312 – Constantine the Great is said to have received his famous Vision of the Cross.
710 – Saracen invasion of Sardinia.
939 – Æthelstan, the first King of England, died and was succeeded by his half-brother, Edmund I.
1275 – Traditional founding of the city of Amsterdam.
1524 – Italian Wars: French troops lay siege to Pavia.
1553 – Condemned as a heretic, Michael Servetus is burned at the stake just outside Geneva.
1644 – Second Battle of Newbury in the English Civil War.
1682 – Philadelphia is established in the Colonial American Commonwealth of Pennsylvania.
1795 – The United States and Spain sign the Treaty of Madrid, which establishes the boundaries between Spanish colonies and the U.S.
1806 – The French Army enters Berlin, following the Battle of Jena–Auerstedt.
1810 – United States annexes the former Spanish colony of West Florida.
1838 – Missouri governor Lilburn Boggs issues the Extermination Order, which orders all Mormons to leave the state or be exterminated.
1870 – Marshal François Achille Bazaine surrenders to Prussian forces at the conclusion of the Siege of Metz along with 140,000 French soldiers in one of the biggest French defeats of the Franco-Prussian War.
1904 – The first underground New York City Subway line opens; the system becomes the biggest in United States, and one of the biggest in the world.
1907 – Černová massacre: Fifteen people are killed in the Hungary when a gunman opens fire on a crowd gathered at a church consecration, which leads to protests over the treatment of minorities in Austria-Hungary.
1914 – The British lose their first battleship of World War I: The British super-dreadnought battleship HMS Audacious (23,400 tons) is sunk off Tory Island, north-west of Ireland, by a minefield laid by the armed German merchant-cruiser Berlin. The loss was kept an official secret in Britain until 14 November
1918 (three days after the end of the war). The sinking was witnessed and photographed by passengers on RMS Olympic.
1916 – Battle of Segale: Negus Mikael, marching on the Ethiopian capital in support of his son Emperor Iyasu V, is defeated by Fitawrari abte Giyorgis, securing the throne for Empress Zewditu I.
1922 – A referendum in Rhodesia rejects the country's annexation to the South African Union.
1924 – The Uzbek SSR is founded in the Soviet Union.
1930 – Ratifications exchanged in London for the first London Naval Treaty, signed in April modifying the 1925 Washington Naval Treaty and the arms limitation treaty's modified provisions, go into effect immediately, further limiting the expensive naval arms race among its five signatories.
1936 – Mrs Wallis Simpson obtains her divorce decree nisi, which would eventually allow her to marry King Edward VIII of the United Kingdom, thus forcing his abdication from the throne.
1944 – World War II: German forces capture Banská Bystrica during Slovak National Uprising thus bringing it to an end.
1954 – Benjamin O. Davis, Jr. becomes the first African-American general in the United States Air Force.
1958 – Iskander Mirza, the first President of Pakistan, is deposed in a bloodless coup d'état by General Ayub Khan, who had been appointed the enforcer of martial law by Mirza 20 days earlier.
1961 – NASA tests the first Saturn I rocket in Mission Saturn-Apollo 1.
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1962 – Cold War: Major Rudolf Anderson of the United States Air Force becomes the only direct human casualty of the Cuban Missile Crisis when his U-2 reconnaissance airplane is shot down over Cuba by a Soviet-supplied SA-2 Guideline surface-to-air missile.
1962 – A plane carrying Enrico Mattei, post-war Italian administrator, crashes in mysterious circumstances.
1964 – Ronald Reagan delivers a speech on behalf of the Republican candidate for president, Barry Goldwater. The speech launches his political career and comes to be known as "A Time for Choosing".
1967 – Catholic priest Philip Berrigan and others of the 'Baltimore Four' protest the Vietnam War by pouring blood on Selective Service records.
1971 – The Democratic Republic of the Congo is renamed Zaire
1973 – A 1.4 kg chondrite-type meteorite strikes in Cañon City, Colorado.
1979 – Saint Vincent and the Grenadines gains its independence from the United Kingdom.
1981 – Cold War: The Soviet submarine S-363 runs aground on the east coast of Sweden.
1986 – The British government suddenly deregulates financial markets, leading to a total restructuring of the way in which they operate in the country, in an event now referred to as the Big Bang.
1988 – Cold War: Ronald Reagan suspends construction of the new U.S. Embassy in Moscow due to Soviet listening devices in the building structure.
1991 – Turkmenistan achieves independence from the Soviet Union.
1992 – United States Navy radioman Allen R. Schindler, Jr. is murdered by shipmate Terry M. Helvey for being gay, precipitating debate about gays in the military that resulted in the United States "Don't ask, don't tell" military policy.
1994 – Gliese 229B is the first Substellar Mass Object to be unquestionably identified.
1995 – Former Prime Minister of Italy Bettino Craxi is convicted in absentia of corruption.
1997 – Stock Market mini-crash: Stock markets around the world crash because of fears of a global economic meltdown. The Dow Jones Industrial Average plummets 554.26 points to 7,161.15.
1999 – Gunmen open fire in the Armenian Parliament, killing Prime Minister Vazgen Sargsyan, Chairman Karen Demirchyan, and six others.
2014 – Britain withdraws from Afghanistan after the end of Operation Herrick which started on June 20, 2002 after 12 years four months and seven days.
2017 – Catalonia declares independence from Spain.
