Background
The end of the First World War had seen the creation of new countries in central Europe. Two of these – the Second Polish Republic and Lithuania, found themselves at loggerheads, disagreeing on where the border between the two countries actually was, ownership of the Lithuanian capital, Vilnius and disagreement over the future of Lithuania itself. Although both countries were also involved in fighting Soviet forces, tensions continued to worsen between the two countries in 1919 and with the failure of the demarcation lines to resolve the issue, it was inevitable that fighting would break out at some point between the two countries.
Attempts to resolve the situation with the introduction of demarcation lines – the final one being the Foch Line – were to ultimately fail, as neither Poland nor Lithuania were happy with the compromise. This would come to a head at Sejny, a town in what is now North-eastern Poland but at the time, was controlled by Lithuania, much to the Poles displeasure.
Sejny marked on a Polish map dated 1919.
Sejny Uprising
The Lithuanians had reluctantly obeyed the terms of the Foch line and began to move their troops (roughly 350 soldiers) back towards the new, agreed border between Poland and Lithuania. However, they only pulled back as far as Sejny and remained there,
ignoring the terms of the demarcation line, with their intent to stay alarming the local Polish population.
The Lithuanians claiming the terms of the Foch Line may yet be altered and so they wished to protect Lithuanian interests in the area.
In response, the Poles organised a rally on August 12, with 100 of them demanding the area come under Polish control, claiming that: Sjeny had changed handed frequently over the previous years – being controlled by both the Russian and Germans Empires previously - so was an obvious location for matters to escalate further.
A counter protest by Lithuanians also took place on August 17th, proclaiming:
The Lithuanian Prime Minister himself, Mykolas Sleževičius visited Sejny and encouraged his countrymen to defend their lands:
With such inflammatory rhetoric being spouted from both sides, it is little wonder events spiralled so quickly.
Mykolas Sleževičius, Prime Minister of Lithuania
Lt. Adam Rudnicki, leader of the Sejny Uprising, and fellow PMO members. August 1919.
The PMO
Then the local branch of the secret Polish Military Organisation (PMO), coordinated by Polish regular army officers Adam Rudnicki and Wacław Zawadzki, started to organise for an uprising. Estimates of Lithuanian military strength in the area are unclear, but it seems likely there was approximately 1000 soldiers.
Around 1000 Polish partisans joined the PMO members and on August 23, they rose up and captured Sejny, taking 100 Lithuanian prisoners, as well as began assaulting the nearby Lithuanian towns of Lazdijai and Kapčiamiestis.
The combined forces of the Polish Army (800 soldiers) and PMO volunteers routed the Lithuanians (650 men) (500 men). Lithuanians were ordered to evacuate behind the Foch Line on 27th August by the Poles. Rudnicki announced the inclusion of PMO volunteers into the 41st Infantry Regiment on 1st September.
The two factions' representatives agreed to negotiate a precise demarcation line on 5th September; Lithuanian representatives pledged to withdraw by 7th September, The PMO rebels who were still fighting on the Lithuanian side were not helped by Polish regular army units because they refused to cross the Foch Line.
According to Polish sources, there were a total of 37 combat deaths and 70 non-combat injuries in Poland during the Sejny Uprising.
Polish cavalry parade in Sejny.
Aftermath
Poland suppressed the cultural life of the Lithuanians in Sejny after the revolt. Sejny (which had about 300 students) and the nearby villages' Lithuanian schools were shut down. The Sejny Priest Seminary was moved, and the local Lithuanian clergy were expelled. The Lithuanians claimed that the repressions went even further, including a prohibition on the public use of the Lithuanian language and the closure of Lithuanian organisations with a combined membership of 1,300.
The 1919 Sejny events were portrayed by The New York Times as a violent occupation by the Poles in which the Lithuanian residents, teachers, and religious leaders were mistreated and driven out when it reported on the outbreak of new hostilities a year later. According to Polish historian Ossowski, both sides exploited stories and tortured civilians in order to win domestic and international support.
Participants in the Sejny uprising
The revolt made things worse between Poland and Lithuania and made the Lithuanians even less inclined to join the projected Midzymorze union. The Polish coup d'état attempt to remove the Lithuanian government was doomed by the Sejny Uprising. Following the revolt, Lithuanian police and intelligence agencies stepped up their investigation of Polish sympathisers, which led them to quickly discover the upcoming coup. Polish militants were detained in large numbers between 27th August, 1919, and 30th September 1919. Lists of PMO sympathisers were discovered during the investigations, and Lithuanian law enforcement totally suppressed the group.
Summer 1920 saw the return of hostilities over the Suwaki Region. During the Polish-Soviet War, the Lithuanians tried to protect what they claimed to be their new frontiers, established by the Soviet-Lithuanian Peace Treaty of July 1920, as the Polish Army started to withdraw. Lithuania was given Sejny and the adjacent area by the Peace Treaty. Poland refused to accept this agreement. Following it, tensions grew until the Polish-Lithuanian War broke out. Sejny was regularly controlled by Polish soldiers beginning on 22nd September 1920.
The Suwaki Agreement on 7th October 1920, which effectively moved the town back to Poland's side of the border, formalised the status.
Sources:
https://militaryhistorynow.com/2012/11/29/military-multi-tasking-poland-fights-five-wars-at-once/
Buchowski, Stanisław. "Powstanie Sejneńskie 23-28 sierpnia 1919 roku" (in Polish). Gimnazjum Nr. 1 w Sejnach. Retrieved 2007-09-27.
"Pietinės Sūduvos lietuviai už šiaudinės administracinės linijos ir geležinės sienos (1920–1991 m.)". Voruta (in Lithuanian). 27–30 (405–408). ISSN 1392-0677
Photo from 1919. Reproduced in Piotr Łossowski, Konflikt polsko-litewski 1918-1920 (The Polish-Lithuanian Conflict, 1918–1920), Warsaw, Książka i Wiedza, 1995, ISBN 8305127699,
Sejny Uprising 1919. Polish spurt that has been completely forgotten - WielkaHistoria
Wiesław Jan Wysocki