The Treaty of Christmemel () was a treaty signed on 19 June 1431 between Paul von Rusdorf, Grand Master the Teutonic Knights, and à  vitrigaila, Grand Duke of Lithuania. à  vitrigaila was preparing for a war with Poland to defend his claim to the Lithuanian throne and sought allies. The treaty established an anti-Polish alliance and prompted the Knights to invade the Kingdom of Poland, starting the PolishâÂÂTeutonic War (1431âÂÂ35). Lithuania also surrendered Palanga and three miles of the coastline on the Baltic Sea, thus modifying the Treaty of Melno of 1422.
Grand Duke Vytautas died in October 1430 leaving no heir to rule the Grand Duchy of Lithuania. According to the terms of the 1413 Union of Horodà Âo, the Lithuanian nobility pledged not to elect a new Grand Duke without the approval of the Kingdom of Poland. Nevertheless, Lithuanian nobles unilaterally elected à  vitrigaila, brother of King of Poland Jogaila, as their Grand Duke. à  vitrigaila refused to acknowledge fealty to his brother and sought to obtain royal crown, originally intended for Vytautas. Poland and Lithuania began preparations for a war. à  vitrigaila sought allies against Poland and envisioned a grand alliance of the Grand Duchy of Lithuania, Holy Roman Empire, Teutonic Knights, Moldavia, and Golden Horde. Securing Teutonic support was of paramount importance.
The Teutonic Knights were a natural ally of à  vitrigaila as they wanted to undo the PolishâÂÂLithuanian union, established in 1385, which led to their defeat in the 1410 Battle of Grunwald and 1422 Gollub War. à  vitrigaila began talks with the Knights soon after the death of Vytautas and kept sending envoys to Prussia. However, Grand Master Paul von Rusdorf delayed and consulted with the Livonian Order, Sigismund, Holy Roman Emperor, prince-electors, and Pope Martin V. Livonian Order advised to support à  vitrigaila but also not to sever the relationship with Poland, Sigismund and prince-electors supported the alliance, while the pope opposed it. Towards the end of May 1431, Rusdorf personally met with à  vitrigaila but again refused to enter into an agreement motivating that he needed to discuss the proposals with Prussian estates. Rusdorf's reluctance could be explained by Poland's attempts to form a PolishâÂÂTeutonic alliance against à  vitrigaila.
Eventually, the treaty was concluded on 19 June 1431 in Christmemel (present-day SkirsnemunÃÂ). The treaty established a military alliance: if one party was attacked, the other was obliged to defend; war is declared only if both parties agree; any peace agreement would apply to both parties equally; spoils of war would be divided equally. The treaty had no expiration and would survive the deaths of à  vitrigaila and Rusdorf if their heirs confirmed the agreement. The treaty was signed by Rusdorf, Livonian Master Zisse von Rutenberg, and Prussian bishops (notably Johannes Ambundii, Archbishop of Riga, did not participate). On the Lithuanian side it was signed by à  vitrigaila, his brother Lengvenis and cousin Sigismund KÃÂstutaitis, Lithuanian bishops and nobles. Nine Lithuanian nobles were: Castellan of Vilnius Kristinas Astikas, Elder of Vilnius Jurgis Gedgaudas, Elder of Samogitia Mykolas KÃÂsgaila, Voivode of Trakai Jaunius KÃÂsgaila, Castellan of Trakai Sungaila, Grand Marshal of Lithuania Rumbaudas Valimantaitis, Chodko Jurewicz, regent of Navahrudak Petras Mantigirdaitis, court marshal Jonas Goà ¡tautas.
Following the Treaty of Christmemel, the Knights invaded Poland and ravaged the Dobrzyà  Land before suffering a defeat in the Battle of Nakel in September 1431. At the same time Polish army invaded Volhynia and besieged à  vitrigaila in the Lubart's Castle in Lutsk. à  vitrigaila proposed peace and a formal two-year Truce of Staryi Chortoryisk was signed on 1 September.
The treaty of 1431 was confirmed again in Christmemel on 15 May 1432. The second treaty had 50 witnesses, however historians noted that some prominent figures of Lithuanian nobility, including Kristinas Astikas, were missing. This is interpreted as a sign of growing opposition to à  vitrigaila's reign. Indeed, a group of nobles organized a coup and deposed of à  vitrigaila in August 1432. The Knights formally observed the Truce of Staryi Chortoryisk, but also continued to support à  vitrigaila, mostly through the Livonian Order. Their alliance was decisively defeated in the Battle of Wià Âkomierz in September 1435.