Flag of Lithuanian SSR.

The Lithuanian Soviet Socialist Republic (Lithuanian: Lietuvos Tarybų Socialistinė Respublika; Russian: Литовская Советская Социалистическая Республика, Litovskaya Sovetskaya Sotsialisticheskaya Respublika), also known as the Lithuanian SSR, was one of the republics that made up the former Soviet Union. Established on 21 July 1940 as a puppet state during World War II in the territory of the previously independent Republic of Lithuania after it had been occupied by the Soviet army on 16 June 1940 in conformity with the terms of 23 August 1939 Molotov-Ribbentrop Pact, it existed until 1990. Between 1941 and 1944, the German invasion of the Soviet Union caused its de facto dissolution. However, with the retreat of the Germans in 1944–1945, Soviet hegemony was re-established. There had been an unsuccessful attempt to establish a Soviet government in Lithuania by the Bolshevik Red Army in 1918–1919.

World War I
The Lithuanian SSR was first proclaimed on 16 December 1918, by the provisional revolutionary government of Lithuania, formed entirely by the Communist Party of Lithuania. The Lithuanian SSR was supported by the Red Army, but it failed to create a de facto government with any popular support as the Council of Lithuania had successfully done earlier. Two months later on 27 February 1919, it was joined by the Soviet Socialist Republic of Byelorussia and they proclaimed the Lithuanian–Belorussian Soviet Socialist Republic (LBSSR or Litbel), which existed for only six months, until 25 August 1919. The Russian Soviet Federated Socialist Republic officially recognized the Republic of Lithuania by signing the Soviet–Lithuanian Peace Treaty on 12 July 1920, thus ending the existence of the fledgling Soviet Republic. It has been suggested that the failure to conquer Poland in the Polish–Soviet War prevented the Soviets from invading Lithuania and re-establishing a Soviet republic at the time.

World War II and occupation
The Molotov–Ribbentrop Pact of August 1939, between Nazi Germany and the Soviet Union, stated that Lithuania was to be included into the German "sphere of influence", but after the World War II broke out in September 1939 the agreement was amended to transfer Lithuania to the Soviet sphere. This was granted in exchange for Lublin and parts of the Warsaw province of Poland, originally ascribed to the Soviet Union, but by that time already occupied by German forces. The Lithuanian Soviet Socialist Republic was established on 21 July 1940 (after Communist rule was forced upon Lithuania following the Soviet ultimatum and subsequent invasion of 15 June 1940). On 3 August 1940, a hastily formed communist government announced that the Lithuanian SSR would become a part of the Soviet Union, i.e. the 14th constituent republic of the USSR. Its territory was subsequently invaded and occupied by Nazi Germany in June 1941, but with the Baltic offensive, Soviet rule was re-established there in July 1944. After both Soviet occupations mass deportation of the Lithuanians into gulags and other forced settlements ensued.

1940 Soviet map of the Lithuanian SSR
The United States, United Kingdom, and other countries considered the occupation of Lithuania by the USSR illegal, citing the Stimson Doctrine, in 1940, but recognized all borders of the USSR at post-World War II conferences. In spite of this, the United States refused to recognize the annexation of Lithuania or the other Baltic States, by the Soviet Union, at any time of the existence of the USSR.

In addition to the human and material losses suffered due to war, several waves of deportations affected Lithuania. During the mass deportation campaign of 14–18 June 1941, about 12,600 people were deported to Siberia without investigation or trial, 3,600 people were imprisoned, and more than 1,000 were killed. After the Lithuanian SSR was re-established in 1944, an estimated 120,000 to 300,000 Lithuanians were either killed or deported to Siberia and other remote parts of the Soviet Union. The Potsdam Conference of 1945 attributed the Klaipėda Region to the Lithuanian SSR.

Lithuania declared independence as the Republic of Lithuania on 11 March 1990. All legal ties of the Soviet Union's sovereignty over the republic were cut as Lithuania declared the restitution of its independence. The Soviet Union claimed that this declaration was illegal, as Lithuania had to follow the process of secession mandated in the Soviet Constitution if it wanted to leave. Lithuania contended that the entire process by which Lithuania joined the Soviet Union violated both Lithuanian and international law so it was merely reasserting an independence that previously existed.

Iceland immediately recognised Lithuania's independence. Most other countries followed suit after the 1991 Soviet coup d'état attempt, with the government of the remaining USSR (Moscow) recognising Lithuania's independence on 6 September 1991.

Economy
Collectivization in the Lithuanian SSR took place between 1947 and 1952.

The 1990 per capita GDP of the Lithuanian Soviet Socialist Republic was $8,591, which was above the average for the rest of the Soviet Union of $6,871. This was still half or less than half of the per capita GDPs of adjacent countries Norway ($18,470), Sweden ($17,680) and Finland ($16,868). Overall, in the Eastern Bloc, the inefficiency of systems without competition or market-clearing prices became costly and unsustainable, especially with the increasing complexity of world economics. Such systems, which required party-state planning at all levels, ended up collapsing under the weight of accumulated economic inefficiencies, with various attempts at reform merely contributing to the acceleration of crisis-generating tendencies.