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1948 (MCMXLVIII) was a leap year starting on Thursday of the Gregorian calendar, the 1948th year of the Common Era (CE) and Anno Domini (AD) designations, the 948th year of the 2nd millennium, the 48th year of the 20th century, and the 9th year of the 1940s decade.
- January 1
- January 4 – Burma gains its independence from the United Kingdom, becoming an independent republic, named the Union of Burma, with Sao Shwe Thaik as its first President, and U Nu its first Prime Minister.
- January 5
- January 7 – Mantell UFO incident: Kentucky Air National Guard pilot Thomas Mantell crashes while in pursuit of an unidentified flying object.
- January 12 – Mahatma Gandhi begins his fast-unto-death in Delhi, to stop communal violence during the Partition of India.
- January 17 – A truce is declared between nationalist Indonesian and Dutch troops in Java.
- January 22 – British foreign secretary Ernest Bevin proposes the formation of a Western Union between Britain, France, and the Benelux countries, to stand up against the Soviet Union. The Treaty of Brussels is signed March 17 as a consequence, a predecessor to NATO.
- January 26 – Teigin poison case: a man masquerading as a doctor poisons 12 of 16 bank employees of the Tokyo branch of Imperial Bank and takes the money; artist Sadamichi Hirasawa is later sentenced to death for the crime, but is never executed.
- January 29
- January 30
- January 31 – The British crown colony of the Malayan Union, Penang and Malacca form the Federation of Malaya.
- February 1
- February 4 – Ceylon (later known as Sri Lanka) becomes an independent kingdom, within the British Commonwealth.
- February 16 – Miranda, innermost of the large moons of Uranus, is discovered by Gerard Kuiper.
- February 18 – Éamon de Valera, Irish head of government from 1918 to 1932, loses power to an opposition coalition. John A. Costello is appointed Taoiseach by President Seán T. O'Kelly, until 1960.
- February 19 – The Conference of Youth and Students of Southeast Asia Fighting for Freedom and Independence convenes in Calcutta.
- February 21 – The United States stock car racing organization NASCAR is founded by Bill France, Sr. with other drivers.
- February 22 – The first of the Ben Yehuda Street bombings in Jerusalem kills between 49 and 58 civilians, and injures between 140 and 200.
- February 25 – 1948 Czechoslovak coup d'état: Edvard Beneš, President of Czechoslovakia, cedes control of the country to the Communist Party, a day celebrated by that regime as "Victorious February" (Czech: Vítězný únor; Slovak: Víťazný Február) until November 1989.
- February 28
- March 8 – McCollum v. Board of Education: The United States Supreme Court rules that religious instruction in public schools violates the U.S. Constitution.
- March 12 – The Costa Rican Civil War begins.
- March 16 – The largest flood in the history of Brampton, Ontario, occurs.
- March 17
- The Treaty of Brussels is signed by Belgium, France, Luxembourg, the Netherlands and the United Kingdom, providing for economic, social and cultural collaboration and collective self-defence.
- The Hells Angels motorcycle gang is founded in California.
- March 18 – The Round Table Conference convenes in The Hague, Netherlands, to prepare the decolonization process for Aruba and the other Dutch Colonies. Aruba presents the mandate of the Aruban People for Aruba to become an independent country, under the sovereignty of the House of Orange, based on Aruba's first state constitution presented officially since August 1947, and a (4th) member state of the future Dutch Commonwealth.
- March 20
Airplane C-54 at airport Berlin-Tempelhof.
- August 1 – The U.S. Air Force Office of Special Investigations is founded.
- August 3 – Whittaker Chambers appears under subpoena before HUAC, and alleges that several former U.S. Federal officials were communists, including Harry Dexter White and Alger Hiss.
- August 5 – Alger Hiss appears before the HUAC, to deny the allegations of Whittaker Chambers.
- August 10 – August 23 – The Herrenchiemsee Convent prepares the draft for the Basic Law for the Federal Republic of Germany.
- August 12 – Babrra massacre: About 600 unarmed members of the Khudai Khidmatgar movement are shot dead on the orders of the Chief Minister of the North-West Frontier Province, Abdul Qayyum Khan Kashmiri, on Babrra ground in the Hashtnagar region of Charsadda District, North-West Frontier Province (now Khyber Pakhtunkhwa), Pakistan.
- August 13 – Harry Dexter White and Donald Hiss refute allegations of Communism by Whittaker Chambers, before the HUAC.
- August 14 – 1948 Ashes series: Australian batsman Don Bradman, playing his last Test cricket match, against England at The Oval, is bowled by Eric Hollies for a duck; however, "The Invincibles" win the match by an innings and 149 runs, and The Ashes 4–0.
- August 15 – The southern half of Korea is established as the Republic of Korea (South Korea).
- August 17 – The HUAC holds a private session between Alger Hiss and Whittaker Chambers.
- August 18 – The Danube Commission is created by the Belgrade Convention (enters into force 11 May 1949).
- August 20 – Lee Pressman, Nathan Witt, and John Abt, represented by Harold I. Cammer, plead the Fifth Amendment, in response to allegations of Communism by Whittaker Chambers before the HUAC.
- August 23 – The World Council of Churches is established in Amsterdam, the Netherlands.
- August 24 – The first meeting of the charter members of the American Chamber of Commerce in Japan (ACCJ) is held.
- August 25 – The HUAC holds its first-ever televised congressional hearing, featuring "Confrontation Day" between Whittaker Chambers and Alger Hiss.
- August 27 – Whittaker Chambers states that Alger Hiss was a communist on Meet the Press radio.
- September 4 – Queen Wilhelmina of the Netherlands abdicates for health reasons.
- September 5 – Robert Schuman becomes Prime Minister of France.
- September 6 – Juliana is formally inaugurated to succeed her mother, as queen regnant of the Netherlands.
- September 9 – The northern half of Korea is formally declared the Democratic People's Republic of Korea (North Korea), with Kim Il-sung as prime minister.
- September 11 – Muhammad Ali Jinnah, founder and first Governor-General of Pakistan, dies. Pakistan is in a state of shock as it mourns the departure of the father of the nation. The day is a public holiday nationwide.
- September 13–18 – Indian annexation of Hyderabad ("Operation Polo"): The princely state of Hyderabad is invaded by the Indian Armed Forces in a "police action", in the aftermath of Pakistani leader Jinnah's death. The Nizam of Hyderabad surrenders his state, which is amalgamated into the newly-independent Dominion of India; thousands are killed as a result of this event.
- September 13 – Margaret Chase Smith is elected United States Senator, and becomes the first woman to serve in both the U.S. House Of Representatives and the United States Senate.
- September 17 – Lehi members, also known as the Stern Gang, assassinate Swedish count Folke Bernadotte, United Nations Mediator in Palestine, in Jerusalem.
- September 18 – An inaugural motor race is held at Goodwood Circuit, West Sussex, England.
- September 20 – The city of Rabwah is established in Pakistan.
- September 27 – Alger Hiss files a slander suit against Whittaker Chambers, for his August 27 radio statement in the United States.
- September 29 – Laurence Olivier's film of Hamlet opens in the U.S.
- October 5 – The International Union for the Protection of Nature (later known as the International Union for the Conservation of Nature, IUCN) is established in Fontainebleau, France.
- October 6 – 1948 Ashgabat earthquake: A 7.3 Ms earthquake near Ashgabat, Soviet Turkmenistan kills 10,000–110,000.
- October 10 – The R-1 missile on test becomes the first Soviet launch to enter space.
- October 16 – The 57th Street Art Fair in Chicago, the oldest juried art fair in the American Midwest, is founded.
- October 20 – Brandeis University is formally founded in Massachusetts.
- October 26 – Donora Smog of 1948: A killer smog settles into Donora, Pennsylvania.
- October 29 – 1948 Arab–Israeli War: Massacres of Palestinian Arab villagers by the Israel Defense Forces:
- October 30 – A luzzu fishing boat overloaded with passengers capsizes and sinks in the Gozo Channel off Qala, Gozo, Malta, killing 23 of the 27 people on board.
- November 1
- November 2 – 1948 United States presidential election: Democratic incumbent Harry S. Truman defeats Republican Thomas E. Dewey, "Dixiecrat" Strom Thurmond, and Progressive party candidate Henry A. Wallace.
- November 12 – In Tokyo, an international war crimes tribunal sentences seven Japanese military and government officials to death, including General Hideki Tojo, for their roles in World War II.
- November 15 – Louis Stephen St. Laurent becomes Canada's 12th prime minister.
- November 16
- November 17
- November 20 – Geoffrey B. Orbell rediscovers the Takahē, last seen 50 years previously, near Lake Te Anau, New Zealand.
- November 24 – In Venezuela, president Rómulo Gallegos is ousted by a military junta.
- November 27 – The Calgary Stampeders defeat the Ottawa Rough Riders 12–7 before 20,013 fans at Toronto's Varsity Stadium, to win their first Grey Cup, and complete the only perfect season to date in .
- December 1 – José Figueres Ferrer abolishes the army in Costa Rica, making it the first country in history to do so.
- December 2 – The United States House Un-American Activities Committee subpoenas and retrieves the "Pumpkin Papers" from the farm of Whittaker Chambers.
- December 6 – Richard Nixon displays microfilm from the "Pumpkin Papers" to the press.
- December 9 – The United Nations General Assembly adopts the Genocide Convention.
- December 10 – The United Nations General Assembly adopts the Universal Declaration of Human Rights.
- December 11–12 – Malayan Emergency: Batang Kali massacre: Scots Guards shoot 24 Chinese villagers in Malaya.
- December 15 – The United States Department of Justice indicts Alger Hiss, on two counts of perjury.
- December 17 – The Finnish Security Police is established to remove communist leadership from its predecessor, the State Police.
- December 19 – In the American , the Philadelphia Eagles defeat the Chicago Cardinals 7–0, to win the championship.
- December 20
- Indonesian National Revolution: The Dutch military captures Yogyakarta, the temporary capital of the newly formed Republic of Indonesia.
- American economist and former State Department official Laurence Duggan falls to his death, from the 16th story window of his Manhattan office.
- December 23 – Seven Japanese military and political leaders, convicted of war crimes by the International Military Tribunal for the Far East, are executed by Allied occupation authorities, at Sugamo Prison in Tokyo, Japan.
- December 26
- December 28 – A Muslim Brotherhood member assassinates Egyptian Prime Minister Mahmud Fahmi Nokrashi.
- December 30 – The musical Kiss Me, Kate opens for the first of 1,077 performances in New York City.
- December 31 – Arab-Israeli War: Israeli troops drive Egyptians from the Negev.
- The Casimir effect is predicted by Dutch physicist Hendrik Casimir.
- The Fresh Kills Landfill, the world's largest, opens on Staten Island, New York.
- The Oakridge Transit Centre opens in Vancouver, British Columbia.
- The Slovak city Gúta is renamed Kolárovo.
- The Vielha Tunnel is opened, giving access to the Val d'Aran in the Spanish Pyrenees; at this time it is the longest road tunnel in the world.
- The last recorded sighting is made of the Caspian tiger, in Kazakhstan.
- A pack of wolves kills about 40 children in Darovskoy District, in Russia.
- Charles Warrell creates the first I-Spy books in the United Kingdom.
- Rev. W. Awdry's third book, James the Red Engine, is published in the United Kingdom.
- The last edition of the Index Librorum Prohibitorum is published in the Vatican.
- Inspired by World War II fighter planes, Cadillac introduces the first automobile to sport tailfins.
- The inaugural 6 Hours of Watkins Glen sports car endurance race is held in the United States.