Vilnius Region under Lithuanian administration (1939–1940)

As a result of the German-Soviet Invasion of Poland part of Vilnius Region was under Lithuanian administration in the period lasting from the takeover of the city from the occupying Soviet administration on October 27, 1939, to the occupation of all of Lithuania including Vilnius on June 15, 1940.

As a result of the Polish-Lithuanian conflict of 1919–1920, and the Polish-Soviet war of 1919–1921, Vilnius and the surrounding region became part of Poland. Lithuania claimed the region, still considering Vilnius as its historical and constitutional capital. The strained relations between the two countries did not improve until 1938. In the following year, however, Poland became the victim of a joint German-Soviet invasion in September 1939; as a result of the defeat, its territory was divided between the two occupiers. Lithuania didn't join the invasion, but on 19 September 1939 reminded of its right to Vilnius. The Soviets offered Lithuania to cede Vilnius and the Vilnius region in return for placing their military bases on its territory.

The Lithuanians agreed to this proposal. The transfer of Vilnius which was completed on 27 October 1939 was celebrated as an act of historical justice, even though it meant a de facto loss of sovereignty. It also led to the severance of diplomatic relations with the Polish government-in-exile, which regarded Lithuanian rule as an occupation. Lithuania opted against an immediate relocation of its capital to Vilnius, instead, embarking on a policy of Lithuanization, that had a negative impact on the Polish population who made up the majority of the population of the incorporated territory. Particularly the closure of all Polish organisations and the withdrawal of civil rights from a population that Lithuania considered foreign. Despite these measures, the Polish population regarded Lithuanian rule as more favourable than the terror perpetrated by the Germans and Soviets in their respective occupation zones.

In June 1940, the Soviets demanded that Lithuania form a friendly government and allow Soviet troops into the entire territory of the state. This act meant the complete loss of sovereignty and the de facto occupation of the whole of Lithuania by the Soviet Union.