August 20 is the 232nd day of the year (233rd in leap years) in the Gregorian Calendar. There are 133 days remaining.
- 636 – Battle of Yarmuk: Arab forces led by Khalid bin Walid take control of Syria and Palestine away from the Byzantine Empire, marking the first great wave of Muslim conquests and the rapid advance of Islam outside Arabia.
- 917 – Battle of Anchialus: Tsar Simeon I of Bulgaria invades Thrace and drives the Byzantines out.
- 1794 – Battle of Fallen Timbers – American troops force a confederacy of Shawnee, Mingo, Delaware, Wyandot, Miami, Ottawa, Chippewa and Potawatomi warriors into a disorganized retreat.
- 1804 – Lewis and Clark Expedition: The "Corps of Discovery", exploring the Louisiana Purchase, suffers its only death when Sergeant Charles Floyd dies, apparently from acute appendicitis.
- 1833 – Nat Turner leads his revolt against the Southern plantation owners of Southampton County, Virginia
- 1882 – Piotr Ilyitch Tchaikovsky's 1812 Overture debuts in Moscow.
- 1914 – World War I: German forces occupy Brussels.
- 1920 – The first commercial radio station, 8MK (WWJ), begins operations in Detroit, Michigan.
- 1940 – Exiled Russian revolutionary Leon Trotsky is fatally wounded in Mexico City by an assassin's ice-ax. He will die the next day.
- 1955 – In Morocco, a force of Berbers from the Atlas Mountains region of Algeria, raid two rural settlements and kill 77 French nationals.
- 1960 – Senegal breaks from the Mali federation, declaring independence.
- 1968 – 200,000 Warsaw Pact troops and 5,000 tanks invade Czechoslovakia to end the "Prague Spring" of political liberalization.
- 1975 – Viking program: NASA launches the Viking 1 planetary probe toward Mars.
- 1977 – Voyager program: The United States launches the Voyager 2 spacecraft.
- 1982 – Lebanese Civil War: A multinational force lands in Beirut to oversee the PLO withdrawal from Lebanon.
- 1986 – In Edmond, Oklahoma, US Postal employee Patrick Henry Sherrill guns down 14 of his co-workers and then commits suicide.
- 1988 – Peru becomes a member of the Berne Convention copyright treaty.
- 1989- In Beverly Hills, California, Lyle and Erik Menendez shoot and kill their wealthy parents.
- 1991 – Collapse of the Soviet Union:
- 1991 – Estonia declares its independence from the Soviet Union.
- 1991 – More than 100,000 people rally outside the Soviet Union's parliament building protesting the coup that deposed President Mikhail Gorbachev.
- 1993 – After rounds of secret negotiations in Norway, the Oslo Peace Accords were signed, followed by a public ceremony in Washington D.C. the next month.
- 1997 – Souhane massacre in Algeria; over 60 people killed, 15 kidnapped.
- 1998 – The Supreme Court of Canada states Quebec cannot legally secede from Canada without the federal government's approval.
- 1998 – 1998 U.S. embassy bombings: The United States military launches cruise missile attacks against alleged al-Qaida camps in Afghanistan and a suspected chemical plant in Sudan in retaliation for the August 7 bombings of American embassies in Kenya and Tanzania. The al-Shifa pharmaceutical factory in Khartoum is destroyed in the attack.
- 2004 – Talk show host Regis Philbin breaks the record for Most Hours on Camera, with 15,188.
- 1561 – Jacopo Peri, composer (d. 1633)
- 1779 – Jöns Jakob Berzelius, chemist (d. 1848)
- 1833 – Benjamin Harrison, 23rd President of the United States (d. 1901)
- 1881 – Edgar Guest, poet (d. 1959)
- 1890 – H. P. Lovecraft, horror writer (d. 1937)
- 1905 – Jack Teagarden, jazz musician (d. 1964)
- 1905 – Jean Gebser, author, linguist and poet (d. 1973)
- 1908 – Al Lopez, baseball player, manager
- 1910 – Eero Saarinen, architect (d. 1961)
- 1918 – Jacqueline Susann, novelist (d. 1974)
- 1923 – Jim Reeves, country and western singer (d. 1964)
- 1931 – Don King, boxing promoter
- 1935 – Ron Paul, American politician
- 1936 – Hideki Shirakawa, Japanese chemist, Nobel Prize, laureate
- 1940 – Rubén Hinojosa, American politician
- 1941 – Slobodan Milosevic, former president of Serbia and Yugoslabia
- 1942 – Isaac Hayes, singer, songwriter, actor
- 1944 – Rajiv Gandhi, Prime Minister of India (d. 1991)
- 1946 – Connie Chung, journalist
- 1946 – N.R. Narayana Murthy, businessman
- 1948 – Robert Plant, rock and roll singer; Cury, João Jorge
- 1949 – Phil Lynott, rock and roll musician (d. 1986)
- 1951 – Greg Bear, science fiction author
- 1952 – John Hiatt, musician
- 1954 – Al Roker, television broadcaster
- 1955 – Agnes Chan, singer, professor of education, essayist
- 1956 – Joan Allen, actress
- 1965 – KRS-One (Lawrence Krisna Parker), rap music singer
- 1968 – Yuri Shiratori, seiyū
- 1970 – John Carmack, game programmer
- 1973 – Todd Helton, baseball player
Holidays and observances
August 19 – August 21 – July 20 – September 20 — listing of all days