Based on the example of the history of the 1st Czechoslovak Army Corps and the 1st Yugoslav Infantry Brigade created on the territory of the USSR during the Great Patriotic War, there are revealed the peculiarities of the effectiveness of “foreign” military units on the territory of a third country. To date, these issues have not been sufficiently studied in historical science. The source base used in the research includes both published and unpublished documents from the funds of the Russian State Archive of Socio-Political History, the Russian State Military Archive and Central Archive of the Ministry of Defense of the Russian Federation. It is shown that despite similar conditions of the formation and assistance from the USSR, the destinies of the Czechoslovak army corps and the Yugoslav brigade were different. The former became a symbol of Czechoslovakia’s struggle against Nazism and the basis of the new Czechoslovak army, whereas the Yugoslav brigade underwent “reformatting”, “dissolved” in the People’s Liberation Army of Yugoslavia; it did not become the basis of the new Yugoslav army and then practically passed into oblivion. The author concludes that with regard to the effectiveness of the Czechoslovak army corps and the Yugoslav brigade, the decisive role was played by the relationship of their command with the main political participant in the process of creating these military units, on whose will both the achievement of the goals set and their very existence depended; the other factors were indirect.