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1990 ( MCMXC) was a common year starting on Monday of the Gregorian calendar, the 1990th year of the Common Era (CE) and (AD) designations, the 990th year of the Anno Domini 2nd millennium, the 90th year of the 20th century, and the 1st year of the 1990s decade.
Important events of 1990 include the
Reunification of Germany and the unification of Yemen, the formal beginning of the  Human Genome Project (finished in 2003), the launch of the Hubble Space Telescope, the separation of Namibia from South Africa, and the Baltic states declaring independence from the Soviet Union amidst Perestroika. Yugoslavia's communist regime collapses amidst increasing internal tensions and multiparty elections held within its constituent republics result in separatist governments being elected in most of the republics marking the beginning of the breakup of Yugoslavia. Also in this year began the crisis that would lead to the Gulf War in 1991 following the Iraq invasion and the largely internationally unrecognized annexation of Kuwait. This annexation resulted in a crisis in the Persian Gulf involving the issue of the sovereignty of Kuwait and fears by Saudi Arabia over Iraqi aggression against their oil fields near Kuwait. This led to Operation Desert Shield being enacted with an international coalition of military forces being built up on the Kuwaiti-Saudi border with demands for Iraq to peacefully withdraw from Kuwait. Also in this year, Nelson Mandela was released from prison, and Margaret Thatcher resigned as Prime Minister of the United Kingdom after more than 11 years.
1990 was an important year in the
Internet's early history. In the fall of 1990, Tim Berners-Lee created the first web server and the foundation for the World Wide Web. Test operations began around December 20 and it was released outside CERN the following year. 1990 also saw the official decommissioning of the  ARPANET, a forerunner of the Internet system and the introduction of the first content web search engine, Archie, on September 10.
September 14, 1990 saw the first case of successful somatic
gene therapy on a patient.
Due to the
early 1990s recession that began that year and uncertainty due to the collapse of the socialist governments in Eastern Europe, birth rates in many countries stopped rising or fell steeply in 1990. In most western countries the Echo Boom peaked in 1990; fertility rates declined thereafter.
ADtranz low floor tram GT6N in Berlin
March 3 – The International Trans-Antarctic Scientific Expedition, a group of six explorers from six nations, completes the first dog sled crossing of Antarctica.
March 8 – The Nintendo World Championships were held within the Fair Park's Automobile Building, kickstarting an almost year long gaming competition across 29 American cities.
March 10 – Prosper Avril is ousted in a coup in Haiti, eighteen months after seizing power.
March 11 – Singing Revolution: The Lithuanian SSR declares independence from the Soviet Union with the Act of the Re-Establishment of the State of Lithuania
March 11– 13 – The March 1990 Central United States tornado outbreak produces 64 tornadoes across six US states, including four violent F4/ F5 tornadoes. The outbreak leaves 2 dead, 89 injured, and causes over $500 million in damages.
March 12 – Cold War: Soviet soldiers begin leaving Hungary under terms of an agreement to withdraw all Soviet troops by June 1.
March 13 – The Supreme Soviet of the Soviet Union approves changes to the Constitution of the Soviet Union to create a strong U.S.-style presidency. Mikhail Gorbachev is elected to a five-year term as the first-ever President of the Soviet Union on March 15.
March 20 – Ferdinand Marcos's widow, Imelda Marcos, goes on trial for bribery, embezzlement, and racketeering. 
March 21 – After 75 years of South African rule since World War I, Namibia becomes independent.
March 24 – 1990 Australian federal election: Bob Hawke's Labor Government is re-elected with a reduced majority, narrowly defeating the Liberal/ National Coalition led by Andrew Peacock. 
March 26 – The 62nd Academy Awards, hosted by Billy Crystal, are held at the Dorothy Chandler Pavilion in Los Angeles, California, with winning Driving Miss Daisy Best Picture.
March 27 – The United States begins broadcasting Radio y Televisión Martí to Cuba.
March 28 – U.S. President George H. W. Bush posthumously awards Jesse Owens the Congressional Gold Medal.
March 30 – Singing Revolution: After its first free elections on March 18, the Estonian SSR declares the Soviet rule to have been illegal since 1940 and declares a transition period for full independence. March 31 – " The Second Battle of Trafalgar": A massive anti- poll tax demonstration in Trafalgar Square, London, turns into a riot; 471 people are injured, and 341 are arrested.
April 6 – Robert Mapplethorpe's "The Perfect Moment" show of nude and homoerotic photographs opens at the Cincinnati Contemporary Arts Center, in spite of accusations of indecency by Citizens for Community Values.
April 9 – Comet Austin, the brightest comet visible from Earth since 1975, makes its closest approach to the sun.
April 12 – Lothar de Maizière becomes prime minister of East Germany, heading a conservative coalition that favors German reunification.
April 13 – Cold War: The Soviet Union apologizes for the Katyn massacre.
April 14 – Junk bond financier Michael Milken pleaded guilty to fraud-related charges. He agreed to pay US$500 million in restitution and was sentenced on November 21 to 10 years in jail.
April 20 – 17-year-old Christopher Kerze goes missing in Eagan, Minnesota. He remains missing as of February 2021 .
April 21 – Japanese Yoshio Tani, M.Sc murders gold merchant Turkka Elovirta and businessman Juhani Komulainen in Siuntio, Finland, having convinced them to buy a nonexistent 500 kilogram stash of Nazi gold.  
Lebanon hostage crisis: Lebanese kidnappers release American educator Robert Polhill, who had been held hostage since January 1987. Earth Day 20 is celebrated by millions worldwide.
April 25 – Violeta Chamorro is sworn in as President of Nicaragua, the first woman elected (February 25) in her own right as a head of state in the Americas.
April 26: A 7.0 earthquake shakes the Chinese province of Qinghai leaving 126 dead. April 30 – Lebanon hostage crisis: Lebanese kidnappers release American educator Frank H. Reed, who had been held hostage since September 1986.
tried to invade
, where the Parliament of Estonia is located, but was dispersed by Estonians
Joanne Rowling gets the idea for Harry Potter while on a train from Manchester to London Euston railway station. She begins writing which will be completed in Harry Potter and the Philosopher's Stone 1995 and published in 1997. 
June 2 – The Lower Ohio Valley tornado outbreak spawns 88 confirmed tornadoes in Illinois, Indiana, Kentucky, and Ohio, killing 12; 37 tornadoes occur in Indiana, eclipsing the previous record of 21 during the Super Outbreak of April 1974.
June 4 – Violence breaks out in the Kirghiz SSR between the majority Kyrgyz people and minority Uzbeks over the distribution of homestead land.
June 8– 9 – In the Czechoslovakian parliamentary election, Czechoslovakia's first free election since 1946, the Civic Forum wins the most seats but fails to secure a majority.
June 9 – Mega Borg oil spill in the Gulf of Mexico near Galveston, Texas.
June 11 – Sri Lankan Civil War: The Liberation Tigers of Tamil Eelam massacre over 600 unarmed police officers in the Eastern Province.
June 13 – Cold War – The destruction of the Berlin Wall by East Germany officially starts, 7 months after it was opened the previous November. 
June 13– 15 – June 1990 Mineriad: Clashes break out in Bucharest between supporters and opponents of the ruling National Salvation Front.
June 14 – 1990 Panay earthquake: An earthquake measuring 7.1 struck Panay Island in the Philippines, killing 8 and injuring 41. 
June 15 – Dublin Regulation on treatment of applications for right of asylum under European Union law agreed (comes into force 1997).
June 17– 30 – Nelson Mandela tours North America, visiting 3 Canadian and 8 U.S. cities. 
June 19 – The Communist Party of the Russian Soviet Federative Socialist Republic holds its inaugural conference in Moscow. 
June 21 – The 7.4 M w Manjil–Rudbar earthquake affects northern Iran with a maximum Mercalli intensity of X ( Extreme), killing 35,000–50,000, and injuring 60,000–105,000.
June 22 – Cold War: Checkpoint Charlie is dismantled.
June 23 – In Canada, the Meech Lake Accord of 1987 dies after the Manitoba and Newfoundland legislatures fail to approve it ahead of the deadline. June 24 – Kathleen Margaret Brown and Irene Templeton are ordained as priests in St Anne's Cathedral in Belfast, becoming the first female Anglican priests in the United Kingdom.
July 1 – German reunification: East Germany and West Germany merge their economies, the West German Deutsche Mark becoming the official currency of the East also. The Inner German border (constructed 1945) also ceases to function.
July 5 – In Kenya, riots erupt against the Kenya African National Union's monopoly on power.
President of Bulgaria Petar Mladenov resigns over accusations that he ordered tanks to disperse anti-government protests in December 1989. Somali President Siad Barre's bodyguards massacre anti-government demonstrators during a soccer match; 65 people are killed, more than 300 seriously injured.
July 7– 8 – In tennis, Martina Navratilova of the United States wins the 1990 Wimbledon Championships – Women's singles and Stefan Edberg of Sweden wins the 1990 Wimbledon Championships – Men's singles.
July 8 – 1990 FIFA World Cup Final (Association football): West Germany defeats Argentina 1–0 to win the 1990 FIFA World Cup. 
July 9– 11 – The 16th G7 summit is held in Houston, Texas.
July 11 – Terrorists blow up a passenger bus travelling from Kalbajar to Tartar in Azerbaijan. 14 people are killed, 35 wounded. 
July 12 – decided in the Foster v British Gas plc European Court of Justice, a leading case on the definition of the "state" under European law.
July 13 – The Lenin Peak disaster occurs when an earthquake triggers an avalanche in the Pamir Mountains with the loss of 43 lives. 
July 16 – 1990 Luzon earthquake: An earthquake measuring 7.7 kills more than 2,400 in the Philippines. 
July 22 – First round of the Mongolian legislative election, the first multiparty ever held in Mongolia; the Mongolian People's Party wins by a wide margin after the second round of voting on July 29.
July 26 – U.S. President George H. W. Bush signs the Americans with Disabilities Act, designed to protect disabled Americans from discrimination.
July 28 – Alberto Fujimori becomes president of Peru. July 30 – British politician and former Member of Parliament Ian Gow is assassinated by a Provisional Irish Republican Army car bomb outside his home in England.
U.S. President Bush orders U.S. combat planes and troops to
Saudi Arabia to prevent a possible attack by Iraq. Prime Minister of India V. P. Singh announces plan to reserve 49% of civil service jobs for lower- caste Hindus. The plan triggers riots, leaving at least 70 dead by September.
Iraq announces its formal annexation of Kuwait.
The government of Peru announces an austerity plan that results in huge increases in the price of food and gasoline. The plan sets off days of rioting and a national strike on August 21.
Egypt, Syria, and 10 other Arab states vote to send military forces to Saudi Arabia to discourage an invasion from Iraq. A passenger bus, traveling along the route "Tbilisi-Agdam", is blown up; 20 people died and 30 were injured. The organizers of the crime were Armenians A. Avanesian and M. Tatevosian who were brought to criminal trial. 
August 19 – Leonard Bernstein conducts his final concert, ending with Ludwig van Beethoven's Symphony No. 7 performed by the Boston Symphony Orchestra.
August 21 – Gambia, Ghana, Guinea, Nigeria, and Sierra Leone send peacekeepers to intervene in the First Liberian Civil War.
August 22 – U.S. President Bush calls up U.S. military reservists for service in the Persian Gulf Crisis.
August 23 – East Germany and West Germany announce they will unite on October 3. 
Armenian SSR declares its independence from the Soviet Union.
Northern Ireland writer Brian Keenan is released from Lebanon after being held hostage for nearly 5 years. Indonesian commercial television network SCTV was established as the nation's third television station after RCTI, and also debuted as local television channel in Surabaya. During its earlier days, SCTV was the rival for RCTI, the first commercial television network. SCTV began broadcasting nationwide from Jakarta by January 29, 1991.
August 26 – In Sofia, protesters set fire to the headquarters of the governing Bulgarian Socialist Party. August 28 – The Plainfield Tornado (F5 on the Fujita scale) strikes the towns of Plainfield, Crest Hill, and Joliet, Illinois, killing 29 people (the strongest tornado to date to strike the Chicago metropolitan area).
Tim Berners-Lee begins his work on the World Wide Web, 19 months after his seminal 1989 outline of what would become the Web concept. 
October 1 – The rebel Rwandan Patriotic Front invades Rwanda from Uganda, marking the start of the Rwandan Civil War.
October 2 – According to The Civil Aviation of China, two commercial planes collide on the runway at the Baiyun Airport of Guangzhou, Guangdong Province, China. The total death toll is 128; 53 people were wounded, 97 were rescued.
October 3 – Cold War: East Germany and West Germany reunify into a single Germany.
October 4 – Moro conflict: Rebel forces seize two military posts on the island of Mindanao, Philippines before surrendering on October 6.  
October 13 – Lebanese Civil War: Syrian military forces invade and occupy Mount Lebanon, ousting General Michel Aoun's government. This effectively consolidates Syria's 14 year occupation of Lebanese soil.
October 14 – Composer and conductor Leonard Bernstein dies of a heart attack at his home in New York City at the age of 72.
October 21 – The remains of the former Estonian head of state, Konstantin Päts, found in the Tver region in Russia, are brought to Tallinn and buried at state expense in the Metsakalmistu cemetery.  
October 22 – Nizhny Novgorod restores its official name from Gorky, Volga Federal District, Russia.
October 29 – In Norway, the government headed by Prime Minister of Norway Jan P. Syse collapses. October 30 – The first transatlantic fiber optic cable TAT-8 fails, causing a slowdown of Internet traffic between the United States and Europe.