sentences:  The disputed Vilnius region centered on the Lithuanian capital of Vilnius (Polish "Wilno"), which had been founded by the Lithuanian Grand Duke Gediminas in 1323 and had been the Lithuanian capital ever since.Vilnius had been occupied by the Soviets during their summer 1920 offensive.The Soviets returned the region to the Lithuanians because the latter had allowed Soviet troops to move through Lithuanian territory and engaged Polish forces in the disputed territories (see Soviet-Lithuanian Treaty of 1920, and Polish-Lithuanian War).