August 20 is the 232nd day of the year (233rd in leap years) in the Gregorian calendar; 133 days remain until the end of the year.
1707 – The first Siege of Pensacola comes to an end with the failure of the British to capture Pensacola, Florida.
1710 – War of the Spanish Succession: A multinational army led by the Austrian commander Guido Starhemberg defeats the Spanish-Bourbon army.
1775 – The Spanish establish the Presidio San Augustin del Tucson in the town that became Tucson, Arizona.
1794 – Northwest Indian War: United States troops force a confederacy of Shawnee, Mingo, Delaware, Wyandot, Miami, Ottawa, Chippewa, and Potawatomi warriors into a disorganized retreat at the Battle of Fallen Timbers.
1852 – Steamboat Atlantic sank on Lake Erie after a collision, with the loss of at least 150 lives.
1858 – Charles Darwin first publishes his theory of evolution through natural selection in The Journal of the Proceedings of the Linnean Society of London.
1866 – President Andrew Johnson formally declares the American Civil War over.
1882 – Tchaikovsky's 1812 Overture debuts in Moscow, Russia.
1910 – Extreme fire weather in the Inland Northwest of the United States causes many small wildfires to coalesce into the Great Fire of 1910, burning approximately 3 million acres and killing 87 people.
1914 – World War I: Brussels is captured during the German invasion of Belgium.
1920 – The first commercial radio station, 8MK (now WWJ), begins operations in Detroit.
1920 – The National Football League is organized as the American Professional Football Conference in Canton, Ohio 1938 – Lou Gehrig hits his 23rd career grand slam, a record that stood for 75 years until it was broken by Alex Rodriguez.
1940 – In Mexico City, exiled Russian revolutionary Leon Trotsky is fatally wounded with an ice axe by Ramón Mercader. He dies the next day.
1940 – World War II: British Prime Minister Winston Churchill makes the fourth of his famous wartime speeches, containing the line 'Never was so much owed by so many to so few'.
1940 – World War II: The Eighth Route Army launches the Hundred Regiments Offensive, a successful campaign to disrupt Japanese war infrastructure and logistics in occupied northern China.
1944 – World War II: One hundred sixty-eight captured allied airmen, accused by the Gestapo of being 'terror fliers', arrive at Buchenwald concentration camp.
1944 – World War II: The Battle of Romania begins with a major Soviet Union offensive.