Sejny Uprising The Sejny Uprising or Seinai Revolt ( , ) refers to a Polish uprising in the ethnically - mixed area surrounding the town of Sejny ( Lithuanian : " Seinai " ) against the Lithuanian authorities in August 1919 . When German forces , which occupied the territory during World War I , retreated from the area , the administration was handed to the Lithuanians . Trying to prevent an armed conflict between Poland and Lithuania , the Entente drew a demarcation line , known as the Foch Line . The line assigned much of the disputed Suwałki ( Suvalkai ) Region to Poland and required the Lithuanian Army to retreat . While the Lithuanians retreated from some areas , they refused to leave Sejny . Polish irregular forces began the uprising on August 23 , 1919 , and soon received support from the regular Polish Army . After several military skirmishes , Polish forces secured Sejny and Lithuanians retreated behind the Foch Line . The uprising did not solve the larger border conflict between Poland and Lithuania over the ethnically - mixed Suwałki Region . Both sides complained about each other ' s repressive measures . The conflict intensified in 1920 , causing military skirmishes of the Polish – Lithuanian War . Sejny changed hands frequently until the Suwałki Agreement of October 1920 , which left Sejny on the Polish side . The uprising undermined the plans of Polish leader Józef Piłsudski , who was planning a coup d ' état in Lithuania to replace the Lithuanian government with a pro - Polish cabinet , which would agree to a union with Poland ( the proposed Międzymorze federation ) . The coup was discovered and stopped as the Sejny Uprising had prompted the Lithuanian intelligence to intensify its investigations of Polish activities in Lithuania . The hostilities in Sejny further strained the Polish – Lithuanian relations . Eventually , Poland and Lithuania reached an agreement on a new border that left Sejny on the Polish side of the border . The Polish – Lithuanian border in the Suwałki Region has remained the same since then ( with the exception of the World War II period ) . Background . During the ages , the lands surrounding the town of Suwałki had been variously part of Lithuanian , Polish , and German borderlands . Since 1569 , during the era of the Polish – Lithuanian Commonwealth , the territory belonged to the Podlaskie Voivodeship and was part of the Kingdom of Poland rather than the Grand Duchy of Lithuania . However , Sejny was a property of Dominican friars from Vilnius . During the 19th century the town was part of Russian - controlled Congress Poland . During World War I the region was captured by the German Empire , which intended to incorporate the area into its province of East Prussia . After the German defeat , the victorious Entente was willing to assign the territory to either the newly independent Poland or Lithuania . The future of the region was discussed at the Paris Peace Conference in January 1919 . The Germans , whose former Ober - Ost administration was preparing to evacuate , initially supported leaving the area to a Polish administration . However , as Poland was becoming an ally of France , German support gradually shifted towards Lithuania . In July 1919 , when the German troops began their slow retreat from the area , they delegated the administration to local Lithuanian authorities . Lithuanian officers and troops , who first arrived to the region in May , began to organize military units in the pre - war Sejny county . According to Russian statistics of 1889 , there were 57 . 8 % of Lithuanians , 19 . 1 % of Poles , and 3 . 5 % of Belarusians in the Suwałki Governorate . It is generally agreed that Lithuanians formed majority in the northern Suwałki Governorate , while Poles concentrated in south . However , Lithuanian and Polish historians continued to disagree over the location of the line that separated the Lithuanian from the Polish majorities . Lithuanians claimed that Sejny and the surrounding area were inhabited primarily by Lithuanians , while the Poles claimed exactly the opposite . The German census of 1916 showed that 51 % of Sejny population was Lithuanian . Demarcation lines . The Conference of Ambassadors drew the first demarcation line between Poland and Lithuania on June 18 , 1919 . The line satisfied no one and Polish troops continued to advance deeper into the Lithuanian - controlled territory . These attacks coincided with signing of the Treaty of Versailles on June 28 and eliminated danger from Germany . Attempting to halt further hostilities , Marshal of France Ferdinand Foch proposed a new line , known as the Foch Line , on July 18 , 1919 . The Foch Line was negotiated with Polish war mission , led by General Tadeusz Jordan - Rozwadowski in Paris , while Lithuanian representatives were not invited . Two major modification were made over the line of June 18 : first , the entire line was moved west to give extra protection to the strategic Warsaw – Saint Petersburg Railway and second , the Suwałki Region , including towns of Sejny , Suwałki , Puńsk , was assigned to Poland . Despite assurances that the lines was just temporary measure to normalize the situation before full negotiations could take place , the southern Foch Line still stands as the border between Lithuania and Poland . On July 26 , the Foch Line was accepted by the Conference of Ambassadors as the provisional border between the two states . Lithuanians were informed about this decision only on August 3 . Neither country was satisfied : both Lithuanian and Polish forces would have to retreat from the Suwałki and Vilnius Regions respectively . Germans , still present in the region , also objected to the line . The Lithuanian forces ( about 350 strong ) left the town of Suwałki by August 7 , but stopped in Sejny and formed a line on the Czarna Hańcza river – Wigry Lake . Lithuanians believed that the Foch Line was not the final decision and that they had the duty to protect Lithuanian outposts in the region . Uprising preparations . On August 12 , 1919 , two days after the Germans retreated from Sejny a Polish meeting in Sejny attracted over 100 delegates from neighboring Polish communities ; the meeting passed a resolution that " only securing the area by Polish Army can solve the problem . " The Sejny branch of the Polish Military Organization ( PMO ) , led by Polish regular army officers Adam Rudnicki and Wacław Zawadzki , began preparing for the uprising on August 16 . PMO members and local militia volunteers numbered some 900 or 1 , 200 men ( sources vary ) . The uprising was scheduled for the night of August 22 to 23 , 1919 . The date was chosen to coincide with the withdrawal of German troops from the town of Suwałki . The Poles hoped to capture the territory up to the Foch Line and move further to the towns of Seirijai , Lazdijai , Kapčiamiestis as far as Simnas . According to the Polish historian Tadeusz Mańczuk , Piłsudski – who was planning a coup d ' état in Kaunas – discouraged the local PMO activists from carrying out the Sejny Uprising . Piłsudski reasoned that any hostilities could leave Lithuanians even more opposed to the proposed union with Poland ( see Międzymorze ) . The local PMO disregarded his recommendations and launched the uprising which , while locally successful , led to the failure of the nationwide coup . On August 17 , a Lithuanian counter - demonstration was staged , whose participants read a recently issued recruitment proclamation of Lithuanian volunteer army : " Citizens ! Our nation is in danger ! To arms ! We shall leave not a single occupant on our lands ! " On August 20 , Prime Minister of Lithuania Mykolas Sleževičius visited Sejny and called on Lithuanians to defend their lands " to the end , however they can , with axes , pitchforks and scythes " . According to Lesčius , at the same time the Lithuanian command in Sejny had only 260 infantry and 70 cavalry personnel , stretched along the long line of defense . There were only 10 Lithuanian guards and 20 clerical staff in the town itself . Mańczuk and Buchowski note that the Polish insurgents estimated the Lithuanian forces at 1 , 200 infantry ( Mańczuk also adds an estimate of 120 cavalry ) , including a 400 - strong garrison in Sejny . Military skirmishes . According to Lithuanian historian Lesčius , the first Polish assault of about 300 PMO members on August 22 was repelled , but the next day Lithuanians were forced to retreat towards Lazdijai . Over 100 Lithuanians were imprisoned in Sejny when their commander Bardauskas sided with the Poles . The Polish insurgents also attacked Lazdijai and Kapčiamiestis , towns on the Lithuanian side of the Foch Line . In early morning of August 25 , Lithuanians counterattacked and recaptured Sejny . Polish sources claim that Lithuanians there were aided by a company of Germans volunteers , but Lithuanian sources assert that it was an excuse used by Rudnicki to explain his defeat . The Lithuanian forces recovered some important documents and property , freed Lithuanian prisoners and , according to Mańczuk , executed several of the PMO fighters they found wounded . On the evening of August 25 , the first regular unit ( 41st Infantry Regiment ) of the Polish army received an order to advance towards Sejny . The Lithuanian forces retreated on the same day when they learned about approaching Polish reinforcements . According to Mańczuk , they based their retreat on an erroneous report about a " large Polish cavalry unit " operating to their rear ; in fact only small groups of Polish partisans operated there . Later the next day , during the afternoon of August 26 , the PMO forces in Sejny were joined by the 41st Infantry Regiment . On August 26 , a large anti - Polish protest took place in Lazdijai , demanding to march on Sejny . The last Lithuanian attempt to retake the town was made on August 28 . The Lithuanians ( about 650 men ) were defeated by the combined forces of the Polish Army ( 800 men ) and PMO volunteers ( 500 men ) . On August 27 , the Poles officially demanded that Lithuanians retreat behind the Foch Line . On September 1 , Rudnicki announced incorporation of PMO volunteers into the 41st Infantry Regiment . During the negotiations on September 5 , it was agreed to settle on a detailed demarcation line ; Lithuanians agreed to retreat by September 7 . The Polish regular army units did not cross the Foch Line , and refused to aid the PMO insurgents still operating on the Lithuanian side . Polish sources give total Polish casualties for the Sejny Uprising as 37 killed in action and 70 wounded . Aftermath . After the uprising , Poland repressed Lithuanian cultural life in Sejny . Lithuanian schools in Sejny ( with some 300 pupils ) and surrounding villages were closed , local Lithuanian clergy evicted , the Sejny Priest Seminary relocated . According to Lithuanians , the repressions were even more widespread , including a ban on public use of the Lithuanian language and the closure of Lithuanian organizations numbering 1 , 300 members . " The New York Times " , reporting on renewed hostilities a year later , described the 1919 Sejny events as a violent occupation , in which the Lithuanian inhabitants , teachers , and religious ministers were maltreated and expelled . Polish historian Łossowski notes that both sides mistreated the civilian population and exaggerated reports to gain internal and foreign support . The uprising contributed to the deterioration of the Polish – Lithuanian relations and further discouraged the Lithuanians from joining Piłsudski ' s proposed Międzymorze federation . The Sejny Uprising doomed the Polish plan to overthrow the Lithuanian government in a coup d ' état . The Lithuanian police and intelligence intensified their investigation of Polish sympathizers and the coup was soon uncovered . Mass arrests of Polish activists continued from August 27 to the end of September 1919 . During the investigations lists of PMO supporters were found and the organisation was completely eliminated in Lithuania . Hostilities over the Suwałki Region resumed in summer 1920 . When Polish Army began to retreat during the course of the Polish – Soviet War , the Lithuanians moved to secure what they claimed to be their new borders , set by the Soviet – Lithuanian Peace Treaty of July 1920 . The Peace Treaty granted Sejny and surrounding area to Lithuania . Poland did not recognize this bilateral treaty . Ensuing tensions grew into the Polish – Lithuanian War . Sejny changed hands frequently until it fell to Polish hands on September 22 , 1920 . The situation was legalized by the Suwałki Agreement of October 7 , 1920 , which returned to town to the Polish side of the border .