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 three independent Baltic countries – Estonia, Latvia, and Lithuania – were invaded and occupied in June 1940 by the Soviet Union, under the Molotov–Ribbentrop Pact between Nazi Germany and the Soviet Union in August 1939, immediately before the outbreak of World War II. The three countries were annexed by the Soviet Union as "constituent republics" in August 1940. Most Western countries did not recognise this annexation, and considered it illegal. On 22 June 1941, Nazi Germany invaded the Soviet Union and within weeks occupied the Baltic territories. In July 1941, the Third Reich incorporated the Baltic territories into its Reichskommissariat Ostland. As a result of the Red Army Baltic Offensive of 1944, the Soviet Union recaptured most of the Baltic states and trapped the remaining German forces in the Courland Pocket until their formal surrender in May 1945.