Vilnius region

Vilnius Region (Lithuanian: Vilniaus kraštas, Polish: Wileńszczyzna, Belarusian: Віленшчына, former English: Wilno or Wilna Region) is the territory in the present-day Lithuania and Belarus that was originally inhabited by ethnic Baltic tribes and was a part of Lithuania proper, but came under East Slavic and Polish cultural influences over time.

The territory included Vilnius, the historical capital of the Grand Duchy of Lithuania. Lithuania, after declaring independence from the Russian Empire, claimed the Vilnius Region based on this historical legacy. Poland argued for the right of self-determination of the local Poles. As a result, throughout the interwar period the control over the area was disputed between Poland and Lithuania. The Soviet Union recognized it as part of Lithuania in the Soviet-Lithuanian Treaty of 1920, but in 1920 it was seized by Poland and became part of short lived puppet state of Central Lithuania, and was subsequently incorporated into the Second Polish Republic.

Direct military conflicts (Polish-Lithuanian War and Żeligowski's Mutiny) were followed up by fruitless negotiations in the League of Nations. After the Soviet invasion of Poland in 1939, as part of the Soviet fulfillment of the Molotov-Ribbentrop pact, the entire region came under Soviet control. About one fifth of the region, including Vilnius, was ceded to Lithuania by the Soviet Union on October 10, 1939 in exchange for Soviet military bases within the territory of Lithuania. The conflict over Vilnius Region was settled after World War II when both Poland and Lithuania came under Soviet and Communist domination and some Poles were repatriated to Poland. Since then, the region became part of the Lithuanian SSR, and since 1990 of modern-day independent Lithuania.

...
Wikipedia