The Polish–Lithuanian War was an undeclared war fought in the aftermath of World War I between newly independent Lithuania and Poland, with fighting mainly in the Vilnius and Suwałki regions, which was part of the Lithuanian Wars of Independence and lasted from May 1919 to 29 November 1920. From the spring of 1920 onward, the conflict happened alongside the wider Polish–Soviet War and was affected by its progress. It was subject to unsuccessful international mediation at the Conference of Ambassadors and the League of Nations.
After World War I, the military and political situation in the region was chaotic, as multiple countries, notably Lithuania, Poland, and Soviet Russia, vied with each other over control of overlapping areas. The Polish–Lithuanian conflict was centered on Vilnius, which the Council of Lithuania declared the capital of the restored Lithuanian state. Control of Vilnius was transferred from Germans to Poles on January 2, 1919, but the Polish paramilitary lost the city to the Bolsheviks on The Polish Army seized Vilnius again on April 19, 1919 and came in contact with the Lithuanian Army fighting in the Lithuanian–Soviet War. Despite the antagonism over Vilnius, the Lithuanian and Polish armies sometimes cooperated when fighting against a common enemy, the Bolsheviks. As Lithuanian–Polish relations worsened, the Entente drew two demarcation lines in hopes to stall further hostilities. The lines did not please either side and were ignored. The first clashes between Polish and Lithuanian soldiers occurred on April 26 and May 8, 1919, near Vievis. With the Polish coup against the Lithuanian government failing in August 1919, the front stabilized until the summer of 1920.
In July 1920, Polish forces retreated due to reverses in the Polish–Soviet War and the Lithuanians followed the retreating troops to secure their lands as delineated in the Soviet–Lithuanian Peace Treaty. However, the Red Army was the first to enter Vilnius. In August 1920, Poland won the Battle of Warsaw and forced the Soviets to retreat. The Polish Army encountered Lithuanian opposition, defending their new borders, which the Polish government considered illegitimate. Thus, the Polish invaded Lithuanian-controlled territory during the Battle of Sejny. Pressured by the League of Nations, Poland signed the Suwałki Agreement on October 7, 1920. The agreement left the Suwałki region on the Polish side and drew a new incomplete demarcation line, which left Vilnius vulnerable to a flanking maneuver.
On October 8, 1920, Polish general Lucjan Żeligowski staged a mutiny, secretly planned and authorized by the Polish chief of state Józef Piłsudski. Żeligowski's forces marched on Vilnius and captured it one day before the Suwałki Agreement was to formally come into effect. Żeligowski proclaimed the creation of the Republic of Central Lithuania with its capital in Vilnius. Their further offensive towards Kaunas, the temporary capital of Lithuania, was halted by the Lithuanians, who thus defended their independence. On November 29, 1920, a ceasefire was signed. Overall, from early 1919 to late 1920, Vilnius would switch rule as many as seven times between Lithuanians, Poles and the Bolsheviks. The Republic of Central Lithuania was incorporated into Poland as the Wilno Voivodeship in 1922. The prolonged mediation by the League of Nations did not change the situation and the status quo was accepted in 1923. In March 1923, the Conference of Ambassadors recognized the armistice line as a de jure Polish–Lithuanian border, awarding Vilnius to Poland. However, the International Court of Justice in The Hague arbitrated in 1931 that Poland broke international law by occupying Vilnius. Lithuania remained adamant regarding its claim to Vilnius as its constitutional capital throughout the whole interwar, and breaking all diplomatic relations with Poland due to the latter's control of the city. Diplomatic relations were only restored as a result of the March 1938 Polish ultimatum to Lithuania. Vilnius was regained by Lithuania on 1939, following the Soviet–Lithuanian Mutual Assistance Treaty, twenty years after losing its capital. However, Lithuania as a whole lost its independence less than a year later following the Soviet ultimatum and occupation of the Baltic states.
Background
Demographic situation
According to the 1897 Russian census, the disputed city of Vilnius had a linguistic breakdown of 40% Jews, and 2% Lithuanian-speaking; and in Trakai county it was 59% Lithuanian- and 11% Polish-speaking. According to the 1916 German census, Poles were the most numerous among all local nationalities and constituted 53% or 53.67% of the city's population, The Polish Self-Defence was formally absorbed into the Polish Army at the end of December. On 2 January 1919, Poles took over Vilnius from the retreating German troops and the fighting with the city's Bolshevik Worker's Soviet (capturing around 1,000 weapons). Nonetheless, the last German soldiers left the city only on January 4. The Lithuanian government failed to organize a defense of Vilnius. Both the Polish and Lithuanian political leadership realized their inability to resist the invading Bolshevik forces. Hence, on , the Council of Lithuania evacuated from Vilnius to Kaunas. After some fighting, the local Polish forces lost Vilnius to the Bolsheviks on January 5. Lithuanians relied on the aid of German troops to stop the Red Army offensive before it reached Kaunas.
At first, the Soviets were successful but this came to a halt in February 1919 due to failures at the battles of Jieznas, Kėdainiai, and Alytus. On February 5, Poland signed an agreement with Germany regulating the withdrawal of the German army from western Belarus and giving the Polish army the opportunity to march eastward. On February 14, 1919, Polish troops came into contact with the Bolshevik army near Vawkavysk. On April 16, 1919, the Polish Army launched a wide-ranging offensive against the Bolsheviks from Lida to Vilnius. On April 19, 1919, the Polish cavalry under Władysław Belina-Prażmowski captured Vilnius. Lithuania was represented by Augustinas Voldemaras and Konstantinas Olšauskas, while Poland was represented by Adam Ronikier. On the other hand, Voldemaras battled negative propaganda that the Council of Lithuania was a German puppet, that Lithuanians harboured pro-Bolshevik attitudes, or that Lithuania was too small and weak to survive without a union with Poland. Under article 87 of the Versailles Treaty, the Principal Allied Powers reserved to themselves the right to determine Poland's eastern frontier.
The war situation and the unstable political situation in both countries did not facilitate contacts between the two governments. Lithuanians protested the presence of Polish troops on Lithuanian territory in early January 1919, but the information reached Warsaw when Vilnius was already in Bolshevik hands. The Poles responded on February 12 by rejecting Lithuanian demands for lands they considered their own, but offered to open direct diplomatic talks. The Lithuanian delegation reached Warsaw on April 18, a day before the entrance of Polish troops to Vilnius.
Meanwhile, Michal Römer, a Lithuanian patriot and veteran of Polish Legions under Piłsudski, went to Kaunas on a mission, and was expected to convince Lithuanian politicians of federation. However, he only found understanding among Lithuanians of Polish culture. On April 17, Lithuanian politicians categorically rejected the federation's proposals.
The Lithuanian delegation, led by Jurgis Šaulys, held talks with Polish Prime Minister Ignacy Jan Paderewski on May 13 and 23, and with Polish Head of State Józef Piłsudski on May 21. The Polish side, strengthened by its possession of Vilnius, insisted on the establishment of a federation, with a common foreign policy, army command, railroad, treasury and post office, promising to establish borders favorable to Lithuania. The Lithuanian side, on the other hand, demanded recognition of an independent Lithuania with Vilnius as its capital. The talks failed, with the only result being the establishment of a Lithuanian consulate in Warsaw, headed by .
May–September 1919: rising tensions
Demarcation lines
The Conference of Ambassadors drew the first demarcation line on June 18, 1919. The line, drawn about west of the Grodno–Vilnius–Daugavpils Railway, was based on the military situation on the ground rather than ethnic composition. Neither Poles nor Lithuanians were content with the line. The Polish Ministry of Foreign Affairs rejected the line as it would require the Polish forces to retreat up to . The line also left the entire Suwałki region, with exception of Augustów, on the Lithuanian side. The Lithuanians protested leaving Vilnius and Grodno under Polish control. As German volunteers were departing from Lithuania and Lithuanian forces were preoccupied with battles against the Soviets in northern Lithuania, Poland ignored the demarcation line and moved its forces on a wide front deeper eastward.
On July 18, Ferdinand Foch proposed the second demarcation line, known as the Foch Line. Initially successful, the Polish Army started retreating after Russian counterattacks in early June 1920. Soon the Soviet forces began to threaten Poland's independence as they reached and crossed the Polish borders. On July 9, Polish Prime Minister Władysław Grabski asked the Allied Powers in the Spa Conference for military assistance in the war with the Soviets. The conference proposed that the Polish forces would withdraw behind the Curzon Line, the Soviet forces would stop to the east of the line, the Lithuanian forces would take control of Vilnius, and all other disputes would be settled via negotiations in London. Grabski opposed the transfer of Vilnius, but under the pressure of British Prime Minister Lloyd George, agreed to the resolution on July 10.
At the same time, the Soviets and Lithuanians negotiated the Soviet–Lithuanian Peace Treaty, which was signed on July 12, 1920. Russia recognized Lithuanian independence and withdrew any territorial claims. The treaty drew the eastern border of Lithuania, which the Lithuanians continued to claim as their de jure state border until World War II. Vilnius Region, including Braslaw (Breslauja), Hrodna (Gardinas), Lida (Lyda), and Vilnius, was recognized to Lithuania. On August 6, after long and heated negotiations, Lithuania and Soviet Russia signed a convention regarding the withdrawal of Russian troops from the recognized Lithuanian territory. However, the troops began to retreat only after the Red Army suffered a heavy defeat in Poland.
Territorial changes
The Bolshevik forces reached Lithuanian territory on July 7, 1920, and continued to push the Polish troops. The Lithuanian Army moved to secure territories abandoned by the retreating Polish forces, reaching Turmantas on July 7, Tauragnai and Alanta on July 9, Širvintos and Musninkai on July 10, Kernavė, Molėtai, and Giedraičiai on July 11, Maišiagala and Pabradė on July 13. On July 13, the Polish command decided to transfer Vilnius to the Lithuanians following the Spa conference's resolution. Lithuanians moved in, but their trains were stopped by Polish soldiers near Kazimieriškės. Polish soldiers didn't receive orders to let Lithuanian troops go through. This delay meant that the Bolsheviks were the first to enter Vilnius on July 14. By the time the first Lithuanian troops entered the city on July 15, it was already secured by the Soviets. Poland sought to have Russians in the city as it would create much fewer complications when the Polish Army counterattacked. Despite the Peace Treaty, the Soviets did not intend to transfer the city to the Lithuanians. Indeed, there were indications that the Soviets planned a coup against the Lithuanian government in hopes to re-establish the Lithuanian SSR.
Despite the setback in Vilnius, the Lithuanians continued to secure territories in the Suwałki Region. They took Druskininkai on July 17, Vištytis, Punsk, Giby, and Sejny on July 19, Suwałki on July 29, Augustów on August 8. The Polish units, afraid of being surrounded and cut off from the main Polish forces, retreated towards Łomża. The Lithuanian authorities started to organize themselves in the regained areas.
Lithuanian neutrality
Poland claimed that Lithuania violated its claim to neutrality in the Polish–Soviet War and in effect became a Soviet ally. A secret clause of the Soviet–Lithuanian Peace Treaty allowed Soviet forces unrestricted movement within the Soviet-recognized Lithuanian territory for the duration of Soviet hostilities with Poland. This clause was of a practical matter: Soviet troops already occupied much of the assigned territory and could not withdraw while hostilities with Poland continued. The League of Nations ended its efforts to mediate the dispute. After Lithuanians seized the Klaipėda Region in January 1923, the League saw recognition of Lithuanian interest in Klaipėda as adequate compensation for the loss of Vilnius. The League accepted the status quo in February 1923 by dividing the neutral zone and setting a demarcation line, which was recognised in March 1923 as the official Polish–Lithuanian border. Lithuania did not recognize this border.
Some historians, notably Alfred E. Senn, have asserted that if Poland had not prevailed in the Polish–Soviet War, Lithuania would have been invaded by the Soviets and would never have experienced two decades of independence.