The Lithuanian Activist Front or LAF was a Lithuanian underground resistance organization established in 1940 after the Soviets occupied Lithuania. Its goal was to free Lithuania and regain its independence. The LAF planned and executed the June uprising and established the short-lived Provisional Government of Lithuania, which disbanded after a few weeks. The Nazi authorities banned the LAF in September 1941. Its role in the three World War II invasions of Lithuania and the massacre of 95% of Lithuania's Jewish population remains ambiguous and the topic of conflicting information and opinion.

The Lithuanian Soviet Socialist Republic, also known as Soviet Lithuania or simply Lithuania, was de facto one of the constituent republics of the Soviet Union between 1940–1941 and 1944–1990. After 1946, its territory and borders mirrored those of today's Republic of Lithuania, with the exception of minor adjustments to its border with Belarus.