1800

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May 15: Napoleon begins crossing the Alps.

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As of March 1 (O.S. February 18), when the Julian calendar acknowledged a leap day and the Gregorian calendar did not, the Julian calendar fell one day further behind, bringing the difference to 12 days until February 28 (O.S. February 16), 1900.

Events

January–March

January 1
- Quasi-War: Action of 1 January 1800 – A naval battle off the coast of Haiti, between four United States merchant vessels escorted by naval schooner Template:USS, and a squadron of armed barges manned by Haitian pirates (known as picaroons), under the command of general André Rigaud, ends indecisively.
- The Dutch East India Company dissolves.
February 7 – A public plebiscite in France confirms Napoleon as First Consul, by a substantial majority.
February 11 – Infrared radiation is discovered by astronomer Sir William Herschel.
February 22 – The Baker rifle, designed by Ezekiel Baker, is selected by the British Board of Ordnance as a new standard.
Template:Anchor March 14 – Papal conclave, 1799–1800: cardinal Barnaba Chiaramonti succeeds Pius VI as Pius VII, the 251st pope. He is crowned on March 21, in Venice.
March 17 – The British Royal Navy ship of the line, Template:HMS, catches fire off the coast of Capraia, with the loss of 673 lives.<ref name="People's Chronology">Template:Cite book</ref>
March 20 – Alessandro Volta describes his new invention, the voltaic pile, the first chemical battery, in a letter to the Royal Society of London.
March 26 – British Royal Navy officer Henry Waterhouse first charts the Antipodes Islands.

April–June

April 2
- Ludwig van Beethoven's Symphony No. 1 premieres at the Burgtheater, in Vienna.
- The Treaty of Constantinople establishes the Septinsular Republic, the first autonomous Greek state since the Fall of the Byzantine Empire.
April 3 – The first voting, albeit indirect, begins in the 1800 United States presidential election as voters in the state of New York make their selections for the New York state legislature, which in turn will vote on October 31 for the state's 12 presidential electors.<ref>Manual of Westchester County, Past and Present (Henry T. Smith, 1898) p. 16</ref> A majority of state legislature members are from the Democratic-Republican Party, rather than the Federalist Party. At the time, only 4 of the 16 U.S. states (Kentucky, Maryland, North Carolina and Virginia) have a popular vote for the presidential electors and the President. The other 12, including New York, have a popular vote for the state legislature, but not for the president. At the time, neither of the major parties have picked their nominees for president and vice president. The result will not be certified until February, 1801.<ref>Narrative and Critical History of America, ed. by Justin Winsor (Houghton Mifflin, 1888) p. 269</ref>
April 6 – War of the Second Coalition: Siege of Genoa – General André Masséna is surrounded by 40,000 Austrian troops under Field Marshal Michael von Melas and blockaded by a strong British squadron under Lord Keith.<ref>Template:Cite book</ref>
April 24 – The U.S. Library of Congress is founded in Washington, D.C.
May 14 – Second Coalition: French forces under General Louis-Alexandre Berthier are halted by 400 Austro-Piedmont soldiers, at Fort Bard in the Aosta Valley.<ref>Template:Cite book</ref>
May 15 – Napoleon and his French army (40,000 men)—not including the field artillery and baggage trains—(35,000 light artillery and infantry, 5,000 cavalry) begin crossing the Alps. He selects the shortest route through the Great St Bernard Pass, and invades after five days traversing the northern region of Italy.
June 2 – The first smallpox vaccination is made in North America, at Trinity, Newfoundland.
June 3 – U.S. President John Adams moves to Washington. Because the President's Mansion is still under construction, President Adams takes up residence at Tunnicliffe's City Hotel near the unfinished U.S. Capitol Building.<ref>Template:Cite web</ref>
June 4 – War of the Second Coalition: Siege of Genoa – The French army is evacuated from Genoa. Marshal André Masséna is allowed to march out, with all the honours of war. A portion of his force joins General Louis-Gabriel Suchet, and the rest is conveyed in British ships to Antibes.<ref>Template:Cite book</ref>
June 14
- War of the Second Coalition: Battle of Marengo – Napoleon defeats the Austrian troops near Marengo, Italy.
- French general Jean-Baptiste Kléber is assassinated in Cairo by Syrian Kurdish Muslim student Suleiman al-Halabi.
June 15 – Convention of Alessandria (Armistice of Marengo): Austria agrees to evacuate much of Italy.
June 19 – War of the Second Coalition: Battle of Höchstädt – General Jean Victor Marie Moreau leads French forces to victory, opening the Danube passageway to Vienna.
June 27 – War of the Second Coalition: Battle of Neuburg – General Claude Lecourbe leads French forces to victory, securing control of the Danube from Austria.

July–September

July 2 – The Union with Ireland Act 1800 is passed by the Parliament of Great Britain; the Irish Parliament passes similar legislation in the following month, uniting the two kingdoms<ref>Template:Cite web</ref><ref>Act of Union 1707.</ref><ref>Template:Cite web</ref> and abolishing the Parliament of Ireland.
July 10 – Fort William College is established by Lord Wellesley, British Governor-General of India, in Calcutta, to promote Bengali, Hindi and other vernaculars of the Indian subcontinent.
August 1 – King George III gives royal assent to the second Act of Union to unite the Kingdom of Great Britain and Kingdom of Ireland (both ruled by him) into the United Kingdom of Great Britain and Ireland, effective on January 1, 1801.<ref>Template:Cite book</ref>
August 30 – The plot by African-American blacksmith and slave Gabriel Prosser to seize Richmond, Virginia, and guide a slave uprising, is thwarted by a massive downpour on the evening that it is set to begin; two other slaves have revealed Prosser's plans to authorities, who have prepared to follow him to the rendezvous point and arrest the conspirators, so that "neither the geographical extent of the plot nor the number of insurgents in the conspiracy was revealed";<ref>Template:Cite book</ref> eventually, 25 slaves, including Prosser, will be captured, tried and hanged.
September 4 – Siege of Malta (1798–1800): The French garrison in Valletta surrenders to British troops, who had been called at the invitation of the Maltese. The islands of Malta and Gozo become the Malta Protectorate.
September 30 – The Convention of 1800, or Treaty of Mortefontaine, is signed between France and the United States of America, ending the Quasi-War.<ref>Template:Cite web</ref>