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 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. 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.

The People's Seimas was a puppet legislature organized in order to give legal sanction the occupation and annexation of Lithuania by the Soviet Union. After the Soviet ultimatum in June 1940, a new pro-Soviet government was formed, known as the People's Government. The new government dismissed the Fourth Seimas and announced elections to the People's Seimas. The elections were heavily rigged, and resulted in a chamber composed entirely of Communists and Communist sympathizers. The new parliament unanimously adopted a resolution proclaiming the Lithuanian Soviet Socialist Republic and petitioned for admission to the Soviet Union as a constituent republic. The Supreme Soviet of the USSR accepted the Lithuanian petition on August 3, 1940. The People's Seimas adopted a new constitution, a close copy of the 1936 Soviet Constitution, on August 25 and renamed itself to the Supreme Soviet of the Lithuanian SSR.