September 18 is the 261st day of the year (262nd in leap years) in the Gregorian calendar; 104 days remain until the end of the year.
324 – Constantine the Great decisively defeats Licinius in the Battle of Chrysopolis, establishing Constantine's sole control over the Roman Empire.
1066 – Norwegian king Harald Hardrada lands with Tostig Godwinson at the mouth of the Humber River and begins his invasion of England.
1180 – Philip Augustus becomes king of France at the age of fifteen
1454 – Thirteen Years' War: In the Battle of Chojnice, the Polish army is defeated by the Teutonic knights.
1544 – The expedition of Juan Bautista Pastene makes landfall in San Pedro Bay, southern Chile, claiming the territory for Spain 1714 – George I arrives in Great Britain after becoming king on August 1.
1739 – The Treaty of Belgrade is signed, whereby Austria cedes lands south of the Sava and Danube rivers to the Ottoman Empire.
1759 – French and Indian War: The Articles of Capitulation of Quebec are signed.
1793 – The first cornerstone of the United States Capitol is laid by George Washington.
1809 – The Royal Opera House in London
opens.
1810 – First Government Junta in Chile. Though supposed to rule only during the Peninsular War in Spain, it is in fact the first step towards independence from Spain, and is commemorated as such.
1812 – The 1812 Fire of Moscow dies down after destroying more than three-quarters of the city. Napoleon returns from the Petrovsky Palace to the Moscow Kremlin, spared from the fire.
1837 – Tiffany & Co. (first named Tiffany & Young) is founded by Charles Lewis Tiffany and Teddy Young in New York City. The store is called a 'stationery and fancy goods emporium'.
1838 – The Anti-Corn Law League is established by Richard Cobden.
1850 – The U.S. Congress passes the Fugitive Slave Act of 1850.
1851 – First publication of The New-York Daily Times, which later becomes The New York Times.
1860 – Second Opium War: Battle of Zhangjiawan: Now heading towards Beijing after having recently occupied Tianjin, the allied Anglo-French force engages and defeats a larger Qing Chinese army at Zhangjiawan.
1862 – The Confederate States celebrate for the first and only time a Thanksgiving Day.
1863 – American Civil War: The Battle of Chickamauga begins between Confederate and Union forces. It involves the second highest amount of casualties for any American Civil War battle apart from Gettysburg.
1864 – American Civil War: John Bell Hood begins the Franklin– Nashville Campaign in an attempt to draw William Tecumseh Sherman back out of Georgia.
1870 – Old Faithful Geyser is observed and named by Henry D. Washburn.
1872 – King Oscar II accedes to the throne of Sweden–Norway.
1873 – The bank Jay Cooke & Company declares bankruptcy, contributing to the Panic of 1873.
1882 – The Pacific Stock Exchange opens.
1895 – The Atlanta Exposition Speech on race relations is delivered by Booker T. Washington.
1898 – The Fashoda Incident triggers the last war scare between Britain and France
1906 – The 1906 Hong Kong typhoon kills an estimated 10,000 people.
1914 – The Irish Home Rule Act becomes law, but is delayed until after World War I.
1919 – Fritz Pollard becomes the first African American to play professional football for a major team, the Akron Pros.
1922 – The Kingdom of Hungary is admitted to the League of Nations.
1927 – The Columbia Broadcasting System goes on the air.
1931 – Imperial Japan instigates the Mukden Incident as a pretext to invade and occupy Manchuria.
1934 – The Soviet Union is admitted to the League of Nations.
1939 – World War II: The Polish government of Ignacy Mościcki flees to Romania.
1939 – World War II: The radio show Germany Calling begins transmitting Nazi propaganda.
1943 – World War II: Adolf Hitler orders the deportation of Danish Jews.
1944 – World War II: The British submarine HMS Tradewind torpedoes Jun'yō Maru, killing 5,600, mostly slave laborers and POWs.
1944 – World War II: Operation Market Garden results in the liberation of Eindhoven.
1944 – World War II: The Battle of Arracourt begins.
1945 – General Douglas MacArthur moves his general headquarters from Manila to Tokyo.
1948 – Margaret Chase Smith of Maine becomes the first woman elected to the United States Senate without completing another senator's term.