The occupation of the Baltic states marked a significant turning point in the geopolitical landscape of Eastern Europe. In June 1940, the Soviet Union issued an ultimatum to Estonia, Latvia, and Lithuania, demanding the establishment of Soviet military bases in these countries. Following a brief period of tense negotiations, the ultimatum was accepted by the governments of the Baltic states, leading to the swift arrival of Soviet military forces. Subsequently, Soviet-aligned puppet governments were installed, and the Soviet players imposed radical changes to the political and social systems of these nations. This occupation would lead to mass repression, deportations, and the incorporation of the Baltic states into the Soviet Union as republics by August 1940.

The region had a complex history of struggle for independence, having gained sovereignty in the aftermath of World War I. The reoccupation by the Soviet Union represented a reversal of their hard-fought independence and set the stage for decades of authoritarian rule. Mobilization of the local bureaucracy and enforcement of Soviet ideology led to widespread changes in culture and governance, cutting off the Baltic states from the West and aligning them closely with Soviet policies.

The response from the West was muted, with many countries focusing on their own matters of self-defense in the early years of World War II. As a result, the Soviet annexation went largely unchallenged initially, though it would later have lasting effects on the Baltic nations' paths towards regaining independence in the late 20th century.