On June 14, 1940, the Soviet government delivered an ultimatum to Lithuania to allow the entry of Soviet troops into Lithuanian territory and for Lithuania to form a new pro-Soviet government. Nine months earlier in October 1939, the two countries had signed the Soviet–Lithuanian Mutual Assistance Treaty which allowed the stationing of 20,000 Soviet troops in a number of bases inside Lithuania.
With Soviet forces already present in the country, on June 15, the Lithuanian government acquiesced to the ultimatum and ended its country’s independence. The Soviets then took control of the country, installed a puppet regime, and held mock elections to the legislature. The new puppet legislature proclaimed the establishment of the Lithuanian Soviet Socialist Republic and petitioned Moscow to be admitted into the Soviet Union. In August 1940, the Soviet government accepted the petition and incorporated Lithuania into the Soviet Union.
The Soviet Union also signed similar mutual assistance agreements with the two other Baltic states, Estonia (September 28, 1939) and Latvia (October 5, 1939) which allowed Soviet forces to occupy strategic locations in these 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 likewise incorporated into the Soviet Union in August 1940.