January

• 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
  • The forces of Russian White Admiral Alexander Kolchak surrender in Krasnoyarsk; the Great Siberian Ice March ensues.
  • The New York State Assembly refuses to seat five duly elected Socialist assemblymen.
• 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
  • Prohibition in the United States begins, with the Eighteenth Amendment to the Constitution coming into effect.
  • The Allies of World War I demand that the Netherlands extradite German Kaiser Wilhelm II, who fled there in 1918.
  • Zeta Phi Beta Sorority, Incorporated, is founded on the campus of Howard University in Washington, D.C.
• January 19
  • The United States Senate votes against joining the League of Nations.
  • The American Civil Liberties Union (ACLU) is founded.
• 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

• 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
  • Estonian War of Independence: The Tartu Peace Treaty is signed, ending the war and recognizing the independence of both the Republic of Estonia and the Russian Socialist Federative Soviet Republic.
  • France occupies Memel.
  • Sayyid Muhammad, Khan of Khiva, abdicates.
• 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

• March 1
  • Hungarian Admiral and statesman Miklós Horthy becomes the Regent of Hungary.
  • The United States Railroad Administration returns control of American railroads to its constituent railroad companies.
• March 7 – The Syrian National Congress proclaims Syria independent, with Faisal I of Iraq as king.
• March 10
  • The world's first peaceful establishment of a social democratic government takes place in Sweden, as Hjalmar Branting takes over as Prime Minister, when Nils Edén leaves office.
  • The Baylor Business Men's Club changes its name, to the Baylor University Chamber of Commerce.
• 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.

April

• April 2 – The German army marches to the Ruhr, to fight the Ruhr Red Army.
• April 4 – 1920 Palestine riots: Violence erupts between Arab and Jewish residents in Jerusalem; 9 are killed, 216 injured.
• April 6 – The short-lived Far Eastern Republic is declared, in eastern Siberia.
• April 11 – Mexican Revolution: Álvaro Obregón flees from Mexico City (during a trial intended to ruin his reputation) to Guerrero, where he joins Fortunato Maycotte.
• April 19–26 – San Remo conference: Representatives of Italy, France, the United Kingdom and Japan meet to determine the League of Nations mandates for administration of territories, following the partitioning of the Ottoman Empire.
• April 19 – Germany and Bolshevist Russia agree to the exchange of prisoners of war.

• April 20
  • Mexican Revolution: Álvaro Obregón announces (in Chilpancingo) that he intends to fight against the rule of Venustiano Carranza.
  • The 1920 Summer Olympics open in Antwerp, Belgium. The Olympic symbols of five interlocking rings and the associated flag are first displayed at the games.
• April 23 – The Grand National Assembly of Turkey is founded by Mustafa Kemal Atatürk, in Ankara. It denounces the government of Sultan Mehmed VI, and announces a temporary constitution.
• April 24 – Polish–Soviet War: Polish and anti-Soviet Ukrainian troops attack the Red Army, in Soviet Ukraine.
• April 26 – The Khorezm People's Soviet Republic is officially created by Bolshevist Russia, as the successor to the Khanate of Khiva.
• April 28 – The Azerbaijan Soviet Socialist Republic is officially created.

May

• May 2 – The first game of Negro National League baseball is played, in Indianapolis, Indiana.
• May 3 – A Bolshevik coup fails, in the Democratic Republic of Georgia.
• May 7
  • Polish–Soviet War: Polish troops occupy Kiev. The government of the Ukrainian People's Republic returns to the city.
  • Mexican Revolution: Venustiano Carranza leaves Mexico City in a large train.
  • Treaty of Moscow (1920): Soviet Russia recognizes the independence of the Democratic Republic of Georgia, only to invade the country six months later.
• May 15 – Russian Revolution: Russian White soldier Maria Bochkareva is executed in Soviet Russia.
• May 16
  • Canonization of Joan of Arc: Over 30,000 people attend the ceremony in Rome, including 140 descendants of Joan of Arc's family. Pope Benedict XV presides over the rite, for which the interior of St. Peter's Basilica in Rome is richly decorated.
  • A referendum in Switzerland favors joining the League of Nations.
• May 17
  • French and Belgian troops leave the cities they have occupied in Germany.
  • The first flight of Dutch air company KLM, from Amsterdam to London, takes place.
• May 19 – Mexican Revolution: Álvaro Obregón's troops enter Mexico City.
• May 20 – Mexican Revolution: Venustiano Carranza arrives in San Antonio Tlaxcalantongo; troops of Rodolfo Herrero attack him at night, and shoot him.
• May 24 – Venustiano Carranza is buried in Mexico City; all of his mourning allies are arrested. Adolfo de la Huerta is elected provisional president.
• May 26 – Ganja revolt: Anti-Soviet opposition in the Azerbaijan SSR launches an abortive revolt in Ganja.
• May 27 – Tomáš Garrigue Masaryk becomes president of Czechoslovakia.
• May 29 – Floods at Louth, Lincolnshire in England kill 23.

June

• June 4 – Treaty of Trianon: Peace is restored between the Allied Powers and Hungary, which loses 72% of its territory.
• June 5 – Bolshevik cavalry break through Polish and Ukrainian lines south of Kiev, precipitating eventual withdrawal.
• June 12 – Polish–Soviet War: The Red Army retakes Kiev.
• June 13
  • Essad Pasha Toptani, nominal ruler of Albania, is assassinated by Avni Rustemi in Paris.
  • The United States Post Office Department rules that children may not be sent via parcel post.
• June 15
  • A new border treaty between Germany and Denmark gives northern Schleswig to Denmark.
  • The Estonian Constituent Assembly adopts the first constitution of Estonia, which will come into effect on December 21 the same year.
• June 22 – Greek Summer Offensive: Greece attacks Turkish troops.

July

• July 1 – Germany declares its neutrality, in the war between Poland and Soviet Russia.
• July 2 – Polish–Soviet War: The Red Army continues its offensive into Poland.
• July 7 – Arthur Meighen becomes Canada's ninth prime minister.
• July 11 – The East Prussian plebiscite is held.
• July 12 – Soviet–Lithuanian Peace Treaty: The Russian Soviet Federative Socialist Republic recognizes independent Lithuania.
• July 13 – The London County Council bars foreigners from council jobs.
• July 19–August 7 – The Second Congress of the Communist International takes place in Saint Petersburg and Moscow; the notorious Twenty-one Conditions are adopted.
• July 20 – The United Kingdom cedes its brief control of the key Black Sea port of Batum, to the Democratic Republic of Georgia.
• July 21 – The Interallied Mission to Poland takes place.
• July 22 – Polish–Soviet War: Poland sues for peace with Bolshevist Russia (which refuses).
• July 24 – Battle of Maysalun: The French defeat the Syrian army, whose leader Yusuf al-'Azma is killed. French troops occupy Damascus, and depose Faisal I of Syria as king.
• July 26 – Mexican Revolution: Pancho Villa takes over Sabina and c