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1790 (MDCCXC)was a common year starting on Friday of the Gregorian calendar and a common year starting on Tuesday of the Julian calendar, the 1790th year of the Common Era (CE) and Anno Domini (AD) designations, the 790th year of the 2nd millennium, the 90th year of the 18th century, and the 1st year of the 1790s decade. As of the start of 1790, the Gregorian calendar was 11 days ahead of the Julian calendar, which remained in localized use until 1923.
- January 8 – United States President George Washington gives the first State of the Union address, in New York City.
- January 11 – The 11 minor states of the Austrian Netherlands, which took part in the Brabant Revolution at the end of 1789, sign a Treaty of Union, creating the United States of Belgium. British Prime Minister William Pitt refuses to recognize the new confederation's independence.
- January 14 – U.S. Secretary of the Treasury Alexander Hamilton submits his proposed plan for payment of American debts, starting with $12,000,000 to pay the foreign debts of the confederation, followed by $40 million for domestic debts, and $21.5 million for the war debts of the states. The plan was narrowly approved 14-12 in the Senate and 34-28 in the House.
- January 26 – Mozart's opera Così fan tutte premieres in Vienna.
- January 30 – The first boat specialized as a rescue lifeboat is tested on the River Tyne in England.
- February 1 – In New York City, the Supreme Court of the United States convenes for the first time.
- February 4 – Louis XVI of France declares to the National Assembly that he will maintain the constitutional laws.
- February 11 – Two Quaker delegates petition the United States Congress for the abolition of slavery.
- February 25 – North Carolina cedes its western territories (modern day Tennessee) to the federal government.
- March 1 – The first United States Census is authorized; it is held later in the year.
- March 4 – France is divided into 83 départements, which cut across the former provinces, in an attempt to dislodge regional loyalties based on noble ownership of land.
- March 6 – The New York legislature consents to the admission to the Union of a new state, Vermont, formed within the boundaries of New York, contingent upon the successful conclusion of negotiations concerning disputed real-estate claims, and the boundary between the two states.
- March 21 – Thomas Jefferson reports to President George Washington in New York, as the new United States Secretary of State.
- April 10 – The United States patent system is established.
- May 13 – Battle of Reval: Gustav III of Sweden sends the battlefleet to eliminate the Russian squadron wintering at Reval (Estonia), but is defeated; 8 Russians, 51 Swedes are killed, 250 captured, and 2 ships are sunk.
- May 17–18 – Battle of Andros: An Ottoman–Algerian fleet destroys the fleet of the Greek privateer Lambros Katsonis.
- May 26 – Congress passes an act to govern the creation of states from the "Southwest Territory", from which Tennessee, Alabama, and Mississippi will be formed.
- May 29 – Rhode Island ratifies the United States Constitution, and becomes the last of the 13 original states to do so.
- June 9 – Royal assent is given to establishment of the port of Milford Haven in Wales.
- June 20 – Compromise of 1790: Thomas Jefferson, James Madison, and Alexander Hamilton come to an agreement: Madison agrees to not be "strenuous" in opposition for the assumption of state debts by the federal government; Hamilton agrees to support the capital site being above the Potomac.
- June 23 – The alleged London Monster is arrested in London; he later receives 40 years for 10 assaults.
- July – Louis XVI of France accepts a constitutional monarchy.
- July 9 – Russo-Swedish War – Second Battle of Svensksund: In a massive Baltic Sea battle of 300 ships, the Swedish Navy captures one third of the Russian galley fleet: 304 Swedes are killed, 3,500 Russians killed and 6,000 captured, 51 Russian galleys and other rowing craft are sunk and 22 are taken.
- July 10 — The U.S. House of Representatives votes, 32-29 to approve creating the District of Columbia from portions of Maryland and Virginia for the eventual seat of government and national capital.
- July 12 – French Revolution: The Civil Constitution of the Clergy is passed. This completes the destruction of the monastic orders, legislating out of existence all regular and secular chapters for either sex, abbacies and priorships.
- July 14 – French Revolution: Citizens of Paris celebrate the unity of the French people and the national reconciliation, in the Fête de la Fédération.
- July 16 – U.S. President George Washington signs the Residence Act into law, establishing a site along the Potomac River as the District of Columbia and the future site of the capital of the United States. The move comes after the bill is narrowly approved on July 1 by the Senate, 14 to 12, and on July 9 by the House, 32 to 29. At the same time, plans are made to move the national capital from New York to Philadelphia until the Potomac River site can be completed.
- July 26 – Alexander Hamilton's Assumption Bill, giving effect to his First Report on the Public Credit, is passed in the United States Congress, allowing the federal government to assume the consolidated debts of the U.S. states.
- July 27 – The Convention of Reichenbach is signed between Prussia and Austria.
- July 31 – Inventor Samuel Hopkins becomes the first to be issued a U.S. patent (for an improved method of making potash).
- August 4 – A newly passed U.S. tariff act creates the system of cutters for revenue enforcement (later named the United States Revenue Cutter Service), the forerunner of the Coast Guard.
- August 14 – The Treaty of Värälä ends the Russo-Swedish War.
- September 25 – The Peking Opera is born, when the Four Great Anhui Troupes introduce Anhui opera to Beijing, in honor of the Qianlong Emperor's 80th birthday.
- September 30 – Leopold II, Holy Roman Emperor starts to rule.
- October 7 – Commissioners appointed by the New York legislature announce the successful conclusion of negotiations between New York and Vermont, concerning disputed real-estate claims, and the consent of New York's legislature to the admission to the Union of the state of Vermont as the 14th State (which was formed within what New York claimed as its territory, under an Order in Council, that King George III issued on July 20, 1764).
- October 20 – The Harmar Campaign ends in a defeat of U.S. Army General Josiah Harmar and Colonel John Hardin by the Western Confederacy of Indians, led by Chief Mihšihkinaahkwa of the Miami tribe and Weyapiersenwah of the Shawnee at Kekionga (now Fort Wayne, Indiana).
- November 24 – France's Constituent Assembly passes a law requiring all Roman Catholic priests to swear an oath of acceptance of the new French Constitution.
- November 27 – U.S. President George Washington and his wife, Martha Washington, arrive in the new temporary U.S. capital, Philadelphia, and take up residence at the President's House located at 524 Market Street.
- December 2 – Holy Roman Empire forces recapture Brussels, bringing an end to the short-lived United States of Belgium and restoring the Austrian Netherlands.
- December 6 – The United States Congress opens its first session in the new temporary U.S. capital in Philadelphia.
- December 10 – The Hawkesbury and Nepean Wars begin in New South Wales, Australia, as a result of deterioration in relations and increasing colonization.
- December 11 – Russo-Turkish War (1787–92): During Alexander Suvorov's storm of Izmail, 26,000 Turkish soldiers lose their lives.