The Czechoslovak Legions (Československé legie in Czech, Československé légie in Slovak, traditionally called Czech Legion in English) were volunteer armed forces composed predominantly of Czechs and Slovaks fighting together with the Entente powers during World War I. Their goal was to win the Allies' support for the independence of Bohemia and Moravia from the Austrian Empire and of Slovak territories of the Kingdom of Hungary (the new country was later named Czechoslovakia), which were then part of the Austro-Hungarian Empire. The Legions originated with small armed units organized from 1914 onwards by volunteer Czechs and Slovaks. Later, many Czechs and Slovaks captured during the war joined these units; with help of émigré intellectuals and politicians (Tomáš Masaryk, Milan Rastislav Štefánik and others) the Legions grew into a force of tens of thousands. The independence of Czechoslovakia was finally obtained in 1918.

After three years of existence as a small brigade in the Imperial Russian Army, the Legion in Russia were created in 1917. Other units had been fighting in France since the war's beginning (including volunteers from the US), and later in Italy and Serbia. Their membership consisted of Czech and Slovak prisoners of war in Russia, Serbia and Italy, and Czech and Slovak emigrants in France and Russia who had already created the "Czech company" in Russia and a unit named "Nazdar" in France in 1914. The Legions were actively involved in many battles of World War I, including Zborov or Bakhmach. They were also heavily involved in the Russian Civil War fighting Bolsheviks, at times controlling much of the Trans-Siberian railway and being indirectly involved in the hasty execution of the Tsar and his family.

World War I (WWI), which was predominantly called the World War or the Great War from its occurrence until 1939 (World War II), and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918. It involved all the world's great powers, which were assembled in two opposing alliances: the Allies (based on the Triple Entente of the United Kingdom, France and Russia) and the Central Powers (originally centred around the Triple Alliance of Germany, Austria-Hungary and Italy; but, as Austria–Hungary had taken the offensive against the agreement, Italy did not enter into the war). These alliances both reorganised (Italy fought for the Allies), and expanded as more nations entered the war. Ultimately more than 70 million military personnel, including 60 million Europeans, were mobilised in one of the largest wars in history. More than 9 million combatants were killed, largely because of enormous increases in lethality of weapons, thanks to new technology, without corresponding improvements in protection or mobility. It was the sixth-deadliest conflict in world history, subsequently paving the way for various political changes such as revolutions in the nations involved.