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1920 (MCMXX)was a leap year starting on Thursday of the Gregorian calendar and a leap year starting on Wednesday of the Julian calendar, the 1920th year of the Common Era (CE) and Anno Domini (AD) designations, the 920th year of the 2nd millennium, the 20th year of the 20th century, and the 1st year of the 1920s decade. As of the start of 1920, the Gregorian calendar was 13 days ahead of the Julian calendar, which remained in localized use until 1923.
- January – First Red Scare: 4,025 suspected communists and anarchists are arrested and held without trial in the United States, following raids in several cities.
- January 1
- Babe Ruth is traded by the Red Sox for $125,000, the largest sum ever paid for a player at that time.
- Bolsheviks increase troops from 4 divisions to 20, along the Polish border.
- January 7
- January 9 – Thousands of onlookers watch as George Polley ("The Human Fly") climbs the Woolworth Building in New York City; he reaches the 30th floor before being arrested.
- January 10 – The League of Nations Covenant enters into force. On January 16, the organization holds its first council meeting, in Paris.
- January 10 – The Treaty of Versailles takes effect, officially ending World War I.
- January 11 – The Azerbaijan Democratic Republic is recognised de facto by European powers in Versailles.
- January 13 – The New York Times ridicules American rocket scientist Robert H. Goddard.
- January 16
- January 19
- January 22 – The Australian Country Party is officially formed, led by Nelson Pollard.
- January 23 – The Netherlands refuses to extradite the German Kaiser.
- January 28 – El Tercio de Extranjeros (the "Regiment of Foreigners"), later the Spanish Legion, is established by decree of King Alfonso XIII of Spain.
- January 30 – The oldest surviving pro wrestling match on film takes place, with Joe Stecher defeating Earl Caddock.
- February 1 – The South African Air Force (SAAF) is established, the second autonomous Air Force in the world, after the Royal Air Force (RAF).
- February 2
- February 7 – Admiral Kolchak and Viktor Pepelyayev are executed by firing squad near Irkutsk.
- February 9 – The Svalbard Treaty, signed by members of the League of Nations in Paris, recognises the sovereignty of Norway over the Arctic archipelago of Svalbard (at this time called Spitzbergen), while giving the other signatories economic rights in the islands.
- February 10 – General Józef Haller first performs Poland's Wedding to the Sea, a symbolic celebration of the restitution of Polish access to the Baltic Sea.
- February 12–24 – Conference of London: Leaders of the United Kingdom, France and Italy meet to discuss the partitioning of the Ottoman Empire.
- February 13 – Switzerland rejoins the League of Nations.
- February 14 – The League of Women Voters is founded in Chicago.
- February 17 – A woman named Anna Anderson tries to commit suicide in Berlin, and is taken to a mental hospital, where she claims she is Grand Duchess Anastasia of Russia.
- February 19 – The United States Senate refuses to ratify the Treaty of Versailles.
- February 20 – 1920 Gori earthquake: An earthquake hits Gori in the Democratic Republic of Georgia, killing 114.
- February 21 – The island province of Marinduque in the Philippines archipelago is founded.
- February 22 – In Emeryville, California, the first dog racing track to employ an imitation rabbit opens.
- February 24 – Adolf Hitler presents his National Socialist Program in Munich to the German Workers' Party (Deutsche Arbeiterpartei), which renames itself as the Nazi Party (Nationalsozialistische Deutsche Arbeiterpartei).
- March 1
- March 7 – The Syrian National Congress proclaims Syria independent, with Faisal I of Iraq as king.
- March 10
- March 13–17 – Wolfgang Kapp and Walther von Lüttwitz's Kapp Putsch (an attempted coup in Germany) briefly ousts the Weimar Republic government from Berlin, but fails due to public resistance and a general strike.
- March 15 – The Ruhr Red Army, a communist army 60,000 men strong, is formed in Germany.
- March 15–16 – Constantinople is occupied by British Empire forces, acting for the Allied Powers against the Turkish National Movement. Retrospectively, the Grand National Assembly of Turkey regards this as the dissolution of the Ottoman regime in Istanbul.
- March 18 – Greece begins using the Gregorian calendar.
- March 19 – The United States Congress refuses to ratify the Treaty of Versailles.
- March 23 – Admiral Miklós Horthy declares that Hungary is a monarchy, without anyone on the throne.
- March 25 – Irish War of Independence: British recruits to the Royal Irish Constabulary begin to arrive in Ireland. They become known from their improvised uniforms as the "Black and Tans".
- March 26 – The German government asks France for permission to use its own troops against the rebellious Ruhr Red Army, in the French-occupied area.
- March 28 – The 1920 Palm Sunday tornado outbreak hits the Great Lakes region and Deep South, of the United States.
- March 29 – Sir William Robertson is promoted to Field Marshal, the first man to rise from private (enlisted 1877) to the highest rank in the British Army.
- September 5 – Presidential elections begin in Mexico.
- September 8 – Gabriele D'Annunzio proclaims the Italian Regency of Carnaro, in the city of Fiume.
- September 12 – The position of Patriarch of the Serbs is re-established as the authority over the Serbian Orthodox Church, almost 156 years to the day after it was abolished by the Ottoman Empire in 1766. 
- September 16 – Wall Street bombing: A bomb in a horse wagon explodes in front of the J. P. Morgan building in New York City, killing 38 and injuring 400.
- September 16 – The Latvian Agrarian Reform Law of 1920 is adopted by the Constitutional Assembly of Latvia.
- September 17 – The is established, as the American Professional Football Association.
- September 20 – The first soldier joins El Tercio de Extranjeros, (the "Regiment of Foreigners", later the Spanish Legion). Under the command of José Millán Astray and Francisco Franco, its first duties are against Rif rebels, in the Spanish protectorate in Morocco.
- September 21 – The Communist Party of Uruguay is founded.
- September 22 – The London Metropolitan Police forms the Flying Squad, a motorised mobile detective patrol unit.
- September 27 – Polish–Soviet War: Bolshevist Russia sues for peace with Poland.
- September 29
- The first domestic radio sets come to stores in the United States; a Westinghouse radio costs $10.
- Adolf Hitler makes his first public political speech, in Austria.
- November 2
- November 11 – The Unknown Warrior is buried in Westminster Abbey.
- November 12 – Italy and the Kingdom of Serbs, Croats and Slovenes sign the Treaty of Rapallo.
- November 13 – The White Army's last units and civilian refugees are evacuated from the Crimea on board 126 ships, the remnants of the Russian Imperial Navy, to Turkey, Tunisia and the Kingdom of Serbs, Croats and Slovenes, accompanied by wide-scale civilian massacres. The total number of evacuees amounts to approximately 150,000 people, of which 20% are civilians.
- November 14 – The Edmonton Symphony Orchestra holds its first concert.
- November 15 – In Geneva, the first assembly of the League of Nations is held.
- November 16 – Queensland and Northern Territory Aviation Services (Qantas) is founded by Hudson Fysh and Paul McGinness.
- November 17 – The council of the League of Nations accepts the constitution for the Free City of Danzig.
- November 20 – Prince Arthur of Connaught is appointed the 3rd Governor-General of the Union of South Africa.
- November 21 – Irish War of Independence: Bloody Sunday: The Irish Republican Army, on the instructions of Michael Collins, shoot dead the "Cairo gang", 14 British undercover agents in Dublin, most in their homes. Later that day in retaliation, the Auxiliary Division of the Royal Irish Constabulary open fire on a crowd at a Gaelic Athletic Association Football match in Croke Park, killing 13 spectators and 1 player, and wounding 60. Three men are shot this night in Dublin Castle "while trying to escape".
- November 28 – Irish War of Independence – Kilmichael Ambush: The flying column of the 3rd Cork Brigade of the Irish Republican Army, led by Tom Barry, ambushes two lorries carrying men of the Auxiliary Division of the Royal Irish Constabulary at Kilmichael, County Cork, killing 17 (with 3 of its men also dying), which leads to official reprisals.