On 20 March 1939, Nazi Germany's foreign minister Joachim von Ribbentrop presented an oral ultimatum to Juozas Urbšys, foreign minister of Lithuania. Germany demanded that Lithuania give up the Klaipėda Region which had been detached from Germany after World War I, or the Wehrmacht would invade Lithuania and the de facto Lithuanian capital Kaunas would be bombed. The Lithuanians had been expecting the demand after years of rising tension between Lithuania and Germany, increasing pro-Nazi propaganda in the region, and continued German expansion. It was issued just five days after the Nazi occupation of Czechoslovakia. The 1924 Klaipėda Convention had guaranteed the protection of the status quo in the region, but the four signatories to that convention did not offer any material assistance. The United Kingdom and France followed a policy of appeasement, while Italy and Japan openly supported Germany, and Lithuania accepted the ultimatum on 23 March 1939. It proved to be the last territorial acquisition for Germany before World War II, producing a major downturn in Lithuania's economy and escalating pre-war tensions for Europe as a whole.

Lithuania Minor, or Prussian Lithuania, is a historical ethnographic region of Prussia, where Prussian Lithuanians lived, now located in Lithuania and the Kaliningrad Oblast of Russia. Lithuania Minor encompassed the northeastern part of the region and got its name from the territory's substantial Lithuanian-speaking population. Prior to the invasion of the Teutonic Knights in the 13th century, the main part of the territory later known as Lithuania Minor was inhabited by the tribes of Skalvians and Nadruvians. The land depopulated during the incessant war between Lithuania and the Teutonic Order. The war ended with the Treaty of Melno and the land was repopulated by Lithuanian newcomers, returning refugees, and the remaining indigenous Baltic peoples; the term Lithuania Minor appeared for the first time between 1517 and 1526.