Abstract: Based on the historical-legal approach, the author studies the national-separatist organization The Forest Brothers that was acting in Lithuania in 1944 - 1953 and was resisting the authorities by means of terrorist acts. The research subject is the set of measures aimed at suppressing nationalist armed groups. Special attention to this research is caused by the necessity to analyze the activities of the Forest Brothers, as in Lithuania since the Post-Soviet period, the actions of nationalists have been heroified, many of whom, during the occupation, served in divisions under the German administration. The purpose of the research is the historical and legal assessment of the activities of separatists in postwar Lithuania. Special attention is given to the operative activities of NKVD aimed at suppressing regional extremism and introducing legal order in postwar Lithuania. Special contribution of the author is the consideration of the problem of national-separatism in Lithuania. It’s been established that to suppress and oppose the Forest Brothers, the bodies of NKVD and the Ministry of the Interior of the Republic developed a set of operative, tactical and ideological measures which helped as early as by the late 1945 to change the situation in the Republic and achieve the permanent annual decrease of destructive actions of nationalists, and since the late 1940s - the early 1950s to fully control the criminogenic situation in the Republic. It’s been established that the set of measures against Lithuanian separatists consisted of five main directions which included the creation of People's Defense Platoons known as “Destroyers”, constant mopping up of forests for extinguishing the resistance zones, intelligence work, large scale amnesty of fighters, deportation of persons who shared the ideas and supported nationalism and separatism. The scientific novelty of the research consists in the critical scientific insight into Lithuanian separatism, its prerequisites and methods of struggling against it by the internal affairs bodies of Lithuania in 1944 - 1953.