The Soviet Union issued an ultimatum to Lithuania demanding that the Red Army be allowed to enter the country and form a pro-Soviet government.

The Soviet Union issued an ultimatum to Lithuania before midnight of 14 June 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.