Kaliningrad is virtually undefended at the moment, due to the manpower strain of the Ukraine war. Lithuania could waltz right in if it wanted, not to mention a full NATO assault.
Until 1945 it was Koenigsberg in East Prussia (known as the birthplace of Immanuel Kant and for a famous mathematical puzzle involving its bridges). Stalin gave half of East Prussia to Poland and annexed the rest to Russia, renaming the city for one of his flunkies. The original king honored by the city’s name was a 13th-century Czech king.
That’s because it’s part of the Russian federation. Lithuania would vanish, or NATO would get nuked if they invaded.