The Soviet Union issued an ultimatum to Lithuania before midnight of June 14, 1940. The Soviets, using a formal pretext, demanded that an unspecified number of Soviet soldiers be allowed to enter the Lithuanian territory and that a new pro-Soviet government (later known as the "People's Government") be formed. The ultimatum and subsequent incorporation of Lithuania into the Soviet Union stemmed from the division of Eastern Europe into the German and Soviet spheres of influence agreed in the Molotov–Ribbentrop Pact of August 1939. Lithuania, along with Latvia and Estonia, fell into the Soviet sphere. According to the Soviet–Lithuanian Mutual Assistance Treaty of October 1939, Lithuania agreed to allow some 20,000 Soviets troops to be stationed at bases within Lithuania in exchange for receiving a portion of the Vilnius Region (previously Polish territory). Further Soviet actions to establish its dominance in its sphere of influence were delayed by the Winter War with Finland and resumed in spring 1940 when Germany was making rapid advances in western Europe. Despite the threat to the country's independence, Lithuanian authorities did little to plan for contingencies and were unprepared for the ultimatum.

With Soviet troops already stationed in the country in accordance with to the Mutual Assistance Treaty, it was impossible to mount effective military resistance. On the 15th of June, Lithuania unconditionally accepted the ultimatum and lost its independence. The Soviets sought to show the world that this was not a military occupation and annexation, but a socialist revolution initiated by the local population demanding to join the Soviet Union. In conformity with this, the Soviets followed semi-legal procedures: they took control of the governmental institutions, installed a puppet government, and carried out show elections to the People's Seimas. During its first session, the Seimas proclaimed the creation of the Lithuanian Soviet Socialist Republic and petitioned to be admitted into the Soviet Union. The petition was officially accepted by the Supreme Soviet of the Soviet Union on 3 August 1940. At the same time, almost identical processes took place in Latvia and Estonia. Lithuania would not regain its independence until the proclamation of the Act of the Re-Establishment of the State of Lithuania on 11 March 1990.