Why did the Soviet Union force Lithuania to accept Soviet military bases in 1939?

The Soviet Union forced Lithuania to accept military bases in 1939 as part of a strategic effort to consolidate its sphere of influence in the Baltic region, which had been established through secret protocols of the German-Soviet Nonaggression Pact.12

Following the outbreak of World War II, the Soviet Union sought to secure its borders as a defensive barrier against potential German aggression. Under a revised secret protocol signed on September 28, 1939, most of Lithuania was placed within the Soviet sphere of influence. In early October, Moscow demanded that the Baltic states sign "mutual assistance" treaties that permitted the stationing of Soviet garrisons on their territory.2134

Isolated and recognizing the futility of military resistance, Lithuania signed the treaty on October 10, 1939. To make the imposition of Soviet military bases more palatable, the Soviet Union returned Vilnius and its surrounding areas to Lithuania—territory the Red Army had occupied during its invasion of eastern Poland. This move was a calculated step toward the eventual annexation of the Baltic states in 1940.14325