During World War I and World War II, several Czechoslovakian military units were established in exile. In WWI, small volunteer formations were later strengthened by Czech & Slovak prisoners-of-war or deserters from the Austro-Hungarian Army and grew into a force of tens of thousands. The majority of Czech & Slovak legionnaires operated in Russia, where they were involved in the Russian Civil War, at times controlling the entire Trans-Siberian railway and several major cities in Siberia. Other organized fighting troops were in France, Italy, and Serbia.

After the occupation of Czechoslovakia in 1939 by Nazi Germany, many professional soldiers emigrated to join the Polish and French armies. In Britain’s Royal Air Force, 2500 Czechs took part in the Battle of Britain. The largest group by far however was in the USSR, where the First Czechoslovak Independent Field Battalion was formed from Czechoslovak refugees living in the Soviet Union, Slovak defectors and prisoners-of-war, and Volhynian Czechs (Soviet citizens of Czech origin) in 1942. After the liberation of large parts of Ukraine in 1943/44, thousands of Volhynian Czechs volunteered to join the Czechoslovak army. At the same time, thousands of Slovak prisoners-of-war, captured or deserted from the Slovak Army, were regrouped and trained as a new paratrooper unit. This all led to the formation of the 1st Czechoslovak Army Corps on 10 April 1944, which consisted of three (later four) infantry brigades and of tank, artillery, engineer, and other support units.