1820

• The United States Congress passes the Missouri Compromise, admitting Maine into the union as a free state, while authorizing the Missouri territory to adopt a constitution with no restrictions on slavery.
• A liberal revolution breaks out in Portugal against the regency of the absentee monarch John VI.
• The Carbonari instigate liberal uprisings in Spain and Naples.
• Tsar Alexander I convenes the Congress of Troppau to consider the means of suppressing the liberal uprisings in Sicily and Spain. Attended by representatives from Russia, Austria, Prussia, Great Britain, and France, the conference is largely a failure due to political bickering, and marks the first serious weakening of the congress system.
• Muhammad Ali, pasha of Egypt, launches an invasion of Sudan.
• Brazil forceably annexes Uruguay.
• Sotho king Moshoeshoe rallies the Zulu to found the kingdom of Lesotho.
• Ecuador declares its independence from Spain.
• 86 free blacks sail from New York to Sierra Leone, Africa, the first colonists of the territory that would later become Liberia.
• The first American missionaries arrive in Hawaii, marking the beginning of the end of Hawaiian autonomy.
• French chemists J.B. Caventou and P.J. Pelletier isolate quinine, long the most effective drug against malaria before the recent development of superior synthetic variants, from the bark of Cinchona plants.
• H.C. Oersted first recognizes a relationship between electricity and magnetism when he finds that a wire carrying an electrical current deflects the needle of a magnetic compass.
• The long rumored continent of Antarctica sighted for the first time by British explorers William Smith and James Bransfield.
• The Venus de Milo is discovered on the Greek island of Mílos.
• American president James Monroe is reelected to a second term in a landslide, losing only a single vote in the electoral college despite a recent economic downturn.
• In response to British pressure, the US Congress passes a law declaring the slave trade a form of piracy punishable by the death penalty, and the US Navy dispatches four vessels to patrol the coast of West Africa for slavers. The campaign would last only four years before the Americans recalled the ships and broke off cooperation with the British.
• In the so-called Cato Street Conspiracy, British dissidents led by Arthur Thistlewood, plan to assassinate the entire British Cabinet. The plot is discovered and Thistlewood is hanged, along with four others.
• Mexican revolutionary Vicente Guerrero wins several minor victories over loyalist general Agustin de Iturbide. Iturbide then switches sides and joins with Guerrero to create the Plan of Iguala, which calls for Mexico to become an independent country with a limited monarchy, the Catholic church as state religion, and equal rights for Spanish and Creoles. The next year Iturbide's army would sweep the country and Iturbide would be proclaimed Emperor.
• Members of Jean Lafitte's Texas pirate colony attack American settlements, causing the U.S. government to dispatch a naval force against him, whereupon Lafitte abondons the colony peacefully.
• Baron Ferdinand Petrovich von Wrangel leads a Russian expedition to explore the Arctic.
• The whaling ship Essex is rammed and sunk by a sperm whale in the Pacific Ocean, an incident that will later inspire Herman Melville's great novel Moby Dick. The survivors are found 94 days later, after a gruelling ordeal that includes starvation and cannibalism.
• John Keats publishes his greatest work, Lamia, Isabella, The Eve of St. Agnes, and Other Poems.
• Percy Bysshe Shelley publishes his masterpiece, Prometheus Unbound.
• Sir Walter Scott publishes his novel Ivanhoe.
• The United States Pharmacopoeia (USP), the authoritative publication on the properties, effects, usage, dosage, strength, and purity of drugs, is published for the first time.

These people were born in 1820:

• American abolitionist and Underground Railroad conductor Harriet Tubman.
• American suffragist Susan B. Anthony.
• English nurse Florence Nightingale.
• German socialist Friedrich Engels, along with Marx one of the two principal founders of Communism.
• Italian king Victor Emmanuel II.
• The father of modern baseball, Alexander Cartwright.
• American general William Tecumseh Sherman.
• Alexander John Cuza, later Alexander John I, the first Prince of Romania.
• Dutch novelist Eduard Douwes Dekker aka Multatuli.
• American publisher and quote compiler John Bartlett.
• French pioneer photographer Nadar (Gaspard-Felix Tournachon).
• American composer George Root.
• British physicist John Tyndall.
• English philosopher Herbert Spenser.
• Slovak philosophical poet Andrej Sládkovic.
• English author Jean Ingelow.
• American arctic explorer Elisha Kent Kane.
• Swedish soprano Jenny Lind.
• English novelist Anne Bronte.
• Black Beauty author Anna Sewell.

These people died in 1820:

• British king George III. He is succeeded by his son, George IV.
• American frontiersman Daniel Boone.
• Haitian king Henri Christophe (Henri I), of suicide.
• Irish statesman and orator Henry Grattan.
• French revolutionary and Napoleon's minister of police Joseph Fouché.
• Argentine revolutionary Manuel Belgrano.
• American revolutionary and Declaration of Independence signatory William Ellery.
• American naval hero Stephen Decatur, in a duel with James Barron.
• Irish lawyer and traitor Leonard MacNally.
• American historical painter Benjamin West.
• British botanist Joseph Banks.
• Scottish explorer of Canada Alexander Mackenzie.
• British economist and statistician, Patrick Colquhoun.

1819 - 1820 - 1821

19th century