The annexation of Estonia by the Soviet Union in 1940 was the result of escalating tensions in Eastern Europe and geopolitical strategies during World War II. This event was rooted in the Molotov-Ribbentrop Pact, signed between the Soviet Union and Nazi Germany in August 1939, which outlined spheres of influence in Eastern Europe. Estonia, along with Latvia and Lithuania, became subject to these negotiations, resulting in the Soviet Union's assertion of influence over the Baltic States.

Following the pact, the Soviet Union positioned itself to enforce its control, demanding that Estonia accept Soviet military bases on its territory in June 1940. Under pressure from the USSR and facing the threat of military action, Estonia's government complied, leading to the resignation of the elected government and the establishment of a pro-Soviet regime. On August 6, 1940, the annexation was officially declared, and Estonia was absorbed into the Soviet Union.

The transition was marked by a swift Sovietization of the country, with significant repressing of opposition, nationalization of industries, and outlawing of political parties. Many Estonians resisted these changes, but they faced severe repercussions, including imprisonment and deportations. This annexation marked the beginning of a long period of Soviet rule that lasted until the end of the 20th century, leading to profound impacts on Estonia's social, political, and economic landscape.