1938 Polish ultimatum to Lithuania

The 1938 Polish ultimatum to Lithuania was an ultimatum delivered to Lithuania by Poland on March 17, 1938. The Lithuanian government had steadfastly refused to have any diplomatic relations with Poland after 1920, protesting the annexation of the Vilnius Region by Poland. As pre-World War II tensions in Europe intensified, Poland perceived the need to secure its northern borders. Five days earlier, Poland, feeling supported by international recognition of the annexation of Austria by Nazi Germany, decided to deliver an ultimatum to Lithuania. The ultimatum demanded that the Lithuanian government unconditionally agree to establish diplomatic relations with Warsaw within 48 hours, and that the terms be finalized before March 31. The establishment of diplomatic relations would mean a de facto renunciation of Lithuanian claims to the region containing its historic capital, Vilnius (known in Polish as Wilno).

Lithuania, preferring peace to war, accepted the ultimatum on March 19. Although diplomatic relations were established as a result of the ultimatum, Lithuania did not agree to recognize the loss of Vilnius de jure. The government of Poland made a similar move against the Czechoslovak government in Prague on September 30, 1938, when it took advantage of the Sudeten Crisis to demand a portion of Zaolzie. On both occasions, Poland used the international crises to address long-standing border disputes.

Lithuania severed its diplomatic ties with Poland after the Polish general Lucjan Żeligowski staged a mutiny in October 1920 by order of Józef Piłsudski. He invaded Lithuanian-held territory, captured the disputed city of Vilnius (known to Poles as Wilno), and established the short-lived Republic of Central Lithuania. This entity was incorporated into Poland in 1922. In demographic terms Vilnius was the least Lithuanian of Lithuanian cities, divided almost evenly between the Polish-speaking population and Jews, with Lithuanian-speaking inhabitants constituting about 2–3% of the population, according to Russian (1897) and German (1916) censuses. Lithuania demanded that Polish troops withdraw behind the line drawn in the Suwałki Agreement, while Poland falsely maintained that it had not authorized Żeligowski's actions. The League attempted to mediate the dispute and Paul Hymans presented concrete proposals to form a federation. However, both sides were unwilling to make compromises and negotiations collapsed in January 1922. In January 1923, Lithuanian troops crossed over to the Allied-held Memelland and staged the "Klaipėda revolt". The Lithuanian occupation of Memel (Klaipėda) was one of the main factors that led to the decision of the Conference of Ambassadors to award Vilnius to Poland in March 1923.

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