1920

1845–1958	Germany [earth sciences]	German naturalist and explorer Alexander von Humboldt lays the basis of modern geography with the publication of Kosmos/Cosmos, in which he arranges geographic knowledge in a systematic fashion.
1920	France [family planning]	Abortion is made illegal in France, to increase the population which has dropped because of World War I.
1920	Russia [family planning]	Abortion is legalized in Russia.
1920	[fiction]	The English writer Agatha Christie publishes The Mysterious Affair at Styles. Her first novel, it introduces her Belgian detective Hercule Poirot.
1920	United Kingdom [food and drink]	Frank Smith launches Smith's Crisps in Britain.
1920	United Kingdom [food and drink]	Brewers in Britain introduce company-labelled beer mats as an advertising tool.
c. 1920	USA [food and drink]	Joseph Krieger manufactures the first commercially available tea bags, in San Francisco, California. Originally intended for caterers, they soon become popular with housewives.
1920	[League of Nations]	Following the establishment of the League of Nations, numerous countries become members, including Argentina (13 January), Switzerland (13 February), Norway (5 March), Denmark (8 March), the Netherlands (10 March), Austria (3 December), Bulgaria, Costa Rica, Finland, and Latvia (16 December), and Albania (17 December).
1920	USA [astronomy]	US physicist Albert Michelson, using a stellar interferometer, measures the diameter of the star Betelgeuse to be 386,160,000 km/241,350,000 mi, which is about 300 times the diameter of the Sun. It is the first time an accurate measurement of the size of a star other than the Sun has been made.
1920	USA [baseball]	The Negro National League, a professional baseball league, is founded in the USA.
1920	Germany [everyday life]	Frei-Sonnenland, the first nudist camp, opens at Motzener See in Germany.
1920	[philosophy]	English philosopher Alfred North Whitehead publishes The Concept of Nature.
1920	[poetry]	The Collected Poems of the English poet Wilfred Owen (killed in World War I) are published posthumously, edited by Siegfried Sassoon.
1920	USA [statistics and demography]	The average life expectancy in the USA is 54 years, up from 49 years in 1901.
1920	United Kingdom [roads]	British roads are classified as A or B, and numbered.
2 January 1920	[births and deaths]	Isaac Asimov, US science fiction writer, born in Petrovichi, Russia (–1992).
8 January 1920	Russia [Russian Civil War (1918–20)]	In the Russian Civil War, the Red Army defeats a White army under Admiral Alexander Kolchak at Krasnoyarsk in south-central Siberia. Kolchak is executed by the Bolsheviks on 7 February.
15 January 1920	United Kingdom [radio]	The Marconi Co. begins the first radio broadcast service in Britain. It is suspended later in the year until 1922, as its signal interferes with transport radio communications.
16 January 1920	USA [food and drink]	Prohibition (the ban on manufacturing, selling, or transporting alcohol) begins in the USA, leading to an increase in the sales of soft drinks and coffee and a thriving bootleg and homebrew industry.
2 February 1920	[treaties]	In a treaty signed at Tartu in Estonia, the Bolsheviks recognize the independence of Estonia and renounce all claims to Russian sovereignty over the Baltic state.
26 February 1920	[administration]	In accordance with the Treaty of Versailles, the League of Nations takes over the Saar area between France and Germany; France takes control of the Saar's coal deposits.
1 March 1920	Hungary [law and government]	The military commander in chief Admiral Nikolaus Horthy is elected regent of Hungary, pending a possible restoration of the Habsburg monarchy.
13 March - 17 March 1920	Germany [revolution]	The ‘Kapp Putsch’ takes place in Germany, the government fleeing when US-born German journalist Wolfgang Kapp and his right wing military supporters seize Berlin, Germany. However, a general strike prevents the conspirators from establishing their authority.
27 March 1920	Russia [Russian Civil War (1918–20)]	The Red Army takes Novorossiysk on the Black Sea, precipitating the collapse of Anton Denikin's White Russian army. General Peter Wrangel takes command of White operations.
23 April 1920	Anatolia [law and government]	A new Turkish assembly opens at Ankara, Anatolia (modern Turkey), which elects the nationalist Mustafa Kemal as its president and proclaims a new constitution, the Law of Fundamental Organization.
25 April - 12 October 1920	Russia, Poland [wars]	A Polish offensive is launched under Joseph Pilsudski, which aims to capture the Ukraine. The action begins the Polish–Russian War.
5 May 1920	Latvia, Germany [treaties]	The Treaty of Berlin ends the war between Germany and Latvia and recognizes Latvian independence.
7 May 1920	Poland, Russia [wars]	During the Polish–Russian War, Polish and Ukrainian forces enter Kiev in the Ukraine, but are driven out by Bolshevik forces on 11 June.
18 May 1920	[births and deaths]	John Paul II, pope from 1978, the first non-Italian pope in 456 years, born in Wadowice, Poland.
20 May 1920	Mexico [terrorism]	President Venustiano Carranza of Mexico is assassinated. In response the US government suspends diplomatic relations with Mexico. Adolfo de la Huerta takes office as provisional president of Mexico.
4 June 1920	[treaties]	The Allies (principally France and Britain) agree the Treaty of Trianon with Hungary. It removes various territories from Hungary, imposes limits on Hungary's armed forces, and requires Hungary to pay reparations for damage inflicted by Austro-Hungarian forces during the war.
4 June 1920	United Kingdom, USA [sports]	The US sculler John B Kelly is refused entry to the Henley Regatta, England, because it is deemed that his job as a bricklayer gives him an unfair advantage in competition with ‘gentlemen’. He goes on to win two gold medals for rowing at the Antwerp Olympic Games.
22 June 1920	Greece, Anatolia [wars]	Greece, with the support of Britain, invades Anatolia (modern Turkey) in order to force it to accept the peace dictated by the Allies.
23 July 1920	Kenya, United Kingdom [colonies and mandate]	Britain annexes its East African Protectorate as Kenya Colony (a crown colony).
24 July 1920	[treaties]	The Treaty of St Germain-en-Laye between the Allies (principally Britain and France) and Austria (signed 19 September 1919) comes into force.
28 July 1920	Czechoslovakia, Poland, France [diplomacy]	The Teschen Agreement, which divides the territory disputed between Czechoslovakia and Poland, is signed in Paris, France.
10 August 1920	Anatolia [treaties]	The Treaty of Sèvres is signed, ending the war between the Ottoman Empire and the Allies. Under the terms of the agreement the Empire is broken up. Mesopotamia and Palestine become British mandates and Syria becomes a French mandate. Part of eastern Thrace, Smyrna (modern Izmir), and other territory is awarded to Greece, Rhodes is awarded to Italy, the Hejaz region becomes independent, and the Dardanelles are internationalized.
11 August 1920	Russia, Latvia [treaties]	Russia and Latvia sign the Riga Treaty, in which Russia recognizes Latvia's independence.
14 August - 16 August 1920	Poland, Russia [wars]	Polish forces under Joseph Pilsudski defeat the advancing Russian troops led by Michael Tukhachevski at Warsaw, Poland.
14 August 1920	Kingdom of the Serbs Croats and Slovenes, Czechoslovakia, Romania [diplomacy]	The Kingdom of the Serbs, Croats, and Slovenes and Czechoslovakia make an alliance (and are joined in 1921 by Romania) to form the ‘Little Entente’, a defensive measure against Hungarian revanchism (a policy aimed at regaining lost territories).
29 August 1920	[births and deaths]	Charlie ‘Yardbird’ or ‘Bird’ Parker, US saxophonist, composer and bandleader, born in Kansas City, Kansas, (–1955).
7 September 1920	USA [everyday life]	The first ‘Miss America’ beauty competition is held in Atlantic City, New Jersey; the winner is Miss Margaret Gorman.
17 September 1920	USA [American football]	A new professional American football league, the American Professional Football Association, the forerunner of the National Football League, is formed at Canton, Ohio. It comprises 10 teams (soon extended to 14).
12 October 1920	Russia, Poland [treaties]	Russia and Poland sign a peace treaty to end their war, at Tartu, Estonia, signing a full treaty on 18 March 1921.
2 November 1920	USA [radio]	The Westinghouse Company establishes the world's first commercial radio station, KDKA, in East Pittsburgh, Pennsylvania, run by the US engineer Frank Conrad, inaugurating national radio broadcasting in the USA. The first broadcast is of the presidential election returns.
14 November 1920	Russia, Anatolia [Russian Civil War (1918–20)]	The Russian Red Army takes Sevastopol in the Crimea. With the evacuation of General Peter Wrangel's White forces to Constantinople, Anatolia (modern Turkey), the civil war in Russia is effectively over, with the communists victorious.
15 November 1920	[political events]	Danzig (modern Gdansk in Poland) is proclaimed a free city under the terms of the Treaty of Versailles. In early December its constitutional assembly is proclaimed the city parliament. Close commercial ties with Poland are confirmed.
5 December 1920	Greece [political events]	A plebiscite in Greece following the death of King Alexander favours the return of the former king, Constantine, who abdicated under Allied pressure in 1917. On 19 December he returns to Greece.
27 December 1920	Italy [political events]	Italian troops force the revolutionary nationalist Gabriele D'Annunzio and his troops out of Fiume, Italy.