Year 1812 (MDCCCXII), a leap year, started on Wednesday (link will display the full calendar) of the Gregorian calendar (or a leap year starting on Monday of the 12-day slower Julian calendar).

Events of 1812

January - March

• January 1 - The Allgemeines bürgerliches Gesetzbuch (the Austrian civil code) enters into force in the Austrian Empire.
• February 2 - Russia establishes a fur trading colony at Fort Ross, California.
• February 7 - The last New Madrid Earthquake strikes New Madrid, Missouri, with an estimated moment magnitude of over 8.
• February 11 - Massachusetts governor Elbridge Gerry invents gerrymandering.
• February 12 - Napoleon authorizes the usage of Mesures usuelles, the basis of the Metric System.
• February 27 - Poet Lord Byron gives his first address as a member of the House of Lords, in defense of Luddite violence against Industrialism in his home county of Nottinghamshire.
• March 12 - The Cádiz Cortes, in exile, create the first modern Spanish constitution.
• March 15 - Luddites attack the wool processing factory of Frank Vickerman in West Yorkshire.
• March 26 - An earthquake destroys Caracas, Venezuela.

April - June

• April 4 - U.S. President James Madison enacts a 90-day embargo on trade with the United Kingdom.
• April 30 - Louisiana is admitted as the 18th U.S. state.
• May 11 - John Bellingham assassinates British Prime Minister Spencer Perceval in the lobby of the British House of Commons.
• May 25 - Caracas, Venezuela is hit by a massive earthquake.
• May 25 - Felling mine disaster: A mine explosion at the Felling colliery near Jarrow, England leaves 96 dead.
• May 28 - Russian Field Marshal Mikhail Kutuzov signs the Treaty of Bucharest, ending the Russo-Turkish War, 1806-1812 and making Bessarabia a part of Imperial Russia.
• June 1 - War of 1812: U.S. President James Madison asks the U.S. Congress to declare war on the United Kingdom.
• June 4 - Following Louisiana's admittance as a U.S. state, the territory by that name is renamed the Missouri Territory.
• June 18 - The War of 1812 begins between the United States and the United Kingdom.
• June 24 - Napoleon's Grande Armée crosses the Niemen River and invades Russia (see Patriotic War of 1812).

July - September

• July 12 - Americans invade Canada at Windsor, Ontario.
• July 18 - Russia's Patriotic War - Battle of Klyastitsy: Kulnev defeats Oudinot but sustains a mortal wound.
• July 22 - Peninsular War - Battle of Salamanca: British forces led by Lord Wellington defeat French troops near Salamanca in Spain.
• August 5 - War of 1812: Tecumseh's Indian force ambushes Thomas Van Horne's 200 Americans at Brownstone Creek, causing them to flee and retreat.
• August 12 - Peninsular War: The combined English and Portuguese army under the command of Wellington enters Madrid following the Battle of Salamanca.
• August 16 - War of 1812: American General William Hull surrenders Fort Detroit without a fight to the British Army.
• August 19 - War of 1812: The USS Constitution defeats the British frigate Guerrière off the coast of Nova Scotia. The British shot is said to have bounced off the Constitution's sides, earning her the nickname "Old Ironsides".
• September 7 - Napoleonic Wars - Battle of Borodino: The bloodiest battle of the Napoleonic wars ends in a draw.
• September 14 - Napoleon's troops enter Moscow, which was deliberately set on fire by Muscovites on orders of Fyodor Rostopchin.

Raevsky Battery in the Battle of Borodino.

October - December

• October - The capital of the Commonwealth of Pennsylvania, United States is permanently moved from Lancaster to Harrisburg.
• October 9 - War of 1812: American naval forces under Lieutenant Jesse Duncan Elliott capture two British warships, HMS Detroit and HMS Caledonia.
• October 13 - War of 1812 - Battle of Queenston Heights: As part of the Niagara campaign in Ontario, Canada, United States forces under General Stephen Van Rensselaer are repulsed from invading Canada by British and native troops led by Sir Isaac Brock (although he dies during the battle).
• October 18-October 20 - Second Battle of Polotsk - Russia
• October 19 - Napoleon begins his retreat from Moscow.
• October 24 - Napoleonic Wars - Battle of Maloyaroslavets: An inconclusive encounter between the French vanguard and a Russian force leads Napoleon to decide to retreat along the same line as his advance, with disastrous results.
• November 5 - James Madison defeats DeWitt Clinton in the U.S. presidential election.
• November 7 - Napoleonic Wars - Battle of Vyazma: Napoleon's retreating army is defeated.
• November 10: A general election in the United Kingdom sees victory for the Tory Party under Robert Jenkinson, 2nd Earl of Liverpool.

Undated

• The Bishop James Madison Society is founded at the College of William & Mary, Williamsburg, Virginia.
• The Old Oscar Pepper Distillery (now the Woodford Reserve Distillery), the oldest Kentucky Bourbon distillery, is established along Glenn's Creek in Woodford County, Kentucky.
• The capital of Finland is moved from Turku to Helsinki.

Ongoing events

• Napoleonic Wars (1799-1815)-Peninsular War/Sixth Coalition/Patriotic War of 1812
• War of 1812 (1812-1815)

Births

January - June

• January 6 - Melchora Aquino, Filipino revolutionary hero (d. 1919)
• January 13 - Victor de Laprade, French poet and critic (d. 1883)
• February 3 - William Fraser Tolmie, Scottish-Canadian scientist and politician (d. 1886)
• February 7 - Charles Dickens, English writer (d. 1870)
• February 11 - Alexander Hamilton Stephens, Vice President of the Confederate States of America (d. 1883)
• February 15 - Charles Lewis Tiffany, American jeweler (d. 1902)
• March 1 - Augustus Pugin, English-born architect (d. 1852)
• March 6 - Aaron Lufkin Dennison American watch manufacturer (d. 1895)
• March 22 - Stephen Pearl Andrews, American anarchist and abolitionist (d. 1886)
• April 22 - Solomon Caesar Malan, Swiss-born orientalist (d. 1894)
• April 27
  • Friedrich von Flotow, German composer (d. 1883)
  • William W. Snow, American politician (d. 1886)
• May 7 - Robert Browning, English poet (d. 1889)
• June 9 - Johann Gottfried Galle, German astronomer (d. 1910)
• June 14 - Fernando Wood, New York City Mayor (d. 1881)

July - December

• July 2 - Nathaniel de Rothschild, French wine grower (d. 1870)
• November 14 - Aleardo Aleardi, Italian poet (d. 1878)
• December 24 - Karl Eduard Zachariae, German jurist (d. 1894)
• Mohan Lal (Zutshi), Indian traveler (d. 1877)

Deaths

January - June

• January 23 - Robert Craufurd, British general (mortally wounded in battle) (b. 1764)
• March 7 - Isaac Swainson, English botanist (b. 1746)
• April 20 - George Clinton, 4th Vice President of the United States (b. 1739)
• April 25 - Edmond Malone, Irish scholar (b. 1741)
• May 11 - Spencer Perceval, Prime Minister of the United Kingdom (assassinated) (b. 1762)
• May 12 - Martha Ballard, American diarist and midwife

July - December

• September 19 - Mayer Amschel Rothschild, German banker (b. 1744)
• October 13 - Isaac Brock, British general (killed in battle) (b. 1769)
• December 20 - Sacagawea, Shoshone guide
• December 24 - George Beck, American artist and poet (b. 1749)