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 Soviet–Lithuanian Mutual Assistance Treaty was a bilateral treaty signed between the Soviet Union and Lithuania on October 10, 1939. According to provisions outlined in the treaty, Lithuania would acquire about one fifth of the Vilnius Region, including Lithuania's historical capital, Vilnius, and in exchange would allow five Soviet military bases with 20,000 troops to be established across Lithuania. In essence the treaty with Lithuania was very similar to the treaties that the Soviet Union signed with Estonia on September 28, and with Latvia on October 5. According to official Soviet sources, the Soviet military was strengthening the defenses of a weak nation against possible attacks by Nazi Germany. The treaty provided that Lithuania's sovereignty would not be affected. However, in reality the treaty opened the door for the first Soviet occupation of Lithuania and was described by The New York Times as "virtual sacrifice of independence."