The Soviet Union occupies Lithuania, which will eventually become the Lithuanian Soviet Socialist Republic (SSR).

The Union of Soviet Socialist Republics (USSR), commonly known as the Soviet Union, was a transcontinental country that spanned much of Eurasia from 1922 until it dissolved in 1991. During its existence, it was the largest country by area, extending across eleven time zones and sharing borders with twelve countries, and the third-most populous country. An overall successor to the Russian Empire, it was nominally organized as a federal union of national republics, the largest and most populous of which was the Russian SFSR. In practice, its government and economy were highly centralized. As a one-party state governed by the Communist Party of the Soviet Union (CPSU), it was the flagship communist state. Its capital and largest city was Moscow.

Lithuania, officially the Republic of Lithuania, is a country in the Baltic region of Europe. It is one of three Baltic states and lies on the eastern shore of the Baltic Sea, bordered by Latvia to the north, Belarus to the east and south, Poland to the south, and the Russian semi-exclave of Kaliningrad Oblast to the southwest, with a maritime border with Sweden to the west. Lithuania covers an area of 65,300 km² (25,200 sq mi), with a population of 2.9 million. Its capital and largest city is Vilnius; other major cities include Kaunas, Klaipėda, Šiauliai and Panevėžys. Lithuanians are the titular nation, belong to the ethnolinguistic group of Balts, and speak Lithuanian.

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.