The ultimatum presented by the Soviet Union to Lithuania came against the backdrop of rising tensions in Eastern Europe following the signing of the Molotov-Ribbentrop Pact, which effectively divided Eastern Europe between Nazi Germany and the Soviet Union. As 1940 unfolded, Lithuania, along with its Baltic neighbors Latvia and Estonia, found itself caught in the crosshairs of Soviet expansionism. The Soviet government cited alleged threats against its security and demands for military presence as justification for the ultimatum. Upon receiving the ultimatum, Lithuania had little choice but to accept the terms, despite widespread public outcry and political dissent. On June 15, 1940, Soviet troops entered Vilnius, the capital of Lithuania, marking the start of a Soviet occupation that would last for almost five decades, deeply affecting the national identity and political landscape of the region.

The ultimatum stipulated that Lithuania allow Soviet military installations within its borders and create a new government comprised largely of pro-Soviet members. The swift compliance of the Lithuanian government to the Soviet demands not only demonstrated the military might of the USSR but also highlighted the geopolitical vulnerabilities of the smaller Baltic states during a time of global upheaval. By the end of June 1940, Lithuania had officially been incorporated into the Soviet Union, which would lead to significant cultural and social changes, including the suppression of nationalist movements and the imposition of Soviet ideology.