With the exception of the Klaipėda Region, which became a mandated territory of the League of Nations in 1920 by the Treaty of Versailles and was unified with Lithuania from 1923 to 1939, the area was part of Prussia until 1945. Since 1945, a small portion of Lithuania Minor has been within the borders of modern-day Lithuania and Poland while most of the territory is part of the Kaliningrad Oblast of Russia, which in turn was part of the Soviet Union until December 1991.