On June 14, 1940, the Soviet government issued an ultimatum to Lithuania to allow Soviet troops to enter Lithuanian territory and to form a new pro-Soviet government. Nine months earlier in October 1939, the two countries had signed the Soviet–Lithuanian Mutual Assistance Treaty that allowed the stationing of 20,000 Soviet troops in a number of bases inside Lithuania.
With the presence of Soviet forces already in the country, on June 15, the Lithuanian government acquiesced to the ultimatum and ended the country’s independence. The Soviets took control of the country, installed a puppet regime, and held sham elections to the legislature. The new legislature proclaimed the establishment of the Lithuanian Soviet Socialist Republic and petitioned to be admitted into the Soviet Union. In August 1940, the Soviet government accepted the petition and admitted Lithuania into the Soviet Union.
The Soviet Union signed similar mutual assistance agreements with the other Baltic States, Estonia (September 28, 1939) and Latvia (October 5, 1939) that allowed Soviet forces to occupy specific bases in the two countries. Also in June 1940, Soviet forces occupied Estonia and Latvia; after socialist governments came to power in Soviet-controlled elections held in July 1940, Estonia and Latvia were similarly incorporated into the Soviet Union in August 1940.