The Polish-Lithuanian War refers to the conflict between newly independent nations of Lithuania and the Second Polish Republic that lasted from July 1919 until October 1920.
The war
Following the start of the Polish-Soviet war in 1919 the majority of Lithuanian territory was soon occupied by the Red Army which defeated and pushed back Polish and Lithuanian self-defence units, but shortly afterwards
the Bolsheviks were forced to retreat by the Polish Army.
Lithuania joined the Soviet Union side in the Polish-Soviet War in July 1919. This decision was dictated by a desire to incorporate the city of Wilno (Vilnius) and the neighbouring areas into Lithuania and to a smaller extent by Soviet diplomatic pressure backed by the threat of the Red Army stationed on Lithuania's borders. The new Lithuanian government decided to make Wilno the capital of Lithuania (it was the historical capital of the Grand Duchy of Lithuania), despite it being populated mainly by Poles and Jews (Lithuanians made about 2% of the population according to the Russian census of 1915. This percentage was higher in the neighbouring rural areas, and in several parts of Vilnius region Lithuanians made majority, while in some others majority was Belorussian).
1920 saw Central Lithuania occupied by the Red Army for the second time, although Russia officially recognized the sovereignty of the Lithuanian Soviet Republic over the city. Lenin was probably waiting for the capture of Warsaw prior to occuppying the remainder of Lithuania. However, when the Red Army was defeated in the Battle of Warsaw, the Soviets made the decision to hand over the city back to Lithuania. Despite the agreement, Lithuania seized the southern Suwalki region as well. The Polish military successes in autumn 1920 allowed Poland to close on to the city but it remained still under Lithuanian control. When the Polish army reached the Lithuanian lines (August 26, 1920), a local war was resumed that lasted for several days until the Lithuanians were pushed back.
This made the compromise even harder to achieve and the newly-established state of Lithuania declined to enter into any negotiations on the status of the Vilnius area, claimed it as its capital city and denied any Polish influence over it whatsoever. A cease-fire agreement was signed on October 7 1920, but it did not solve the issue. The Polish commander Jozef Pilsudski ordered his subordinate, General Lucjan Zeligowski, to defect with his '1st Lithuanian-Belarussian Division' and capture the city, without formal declaration of war on Lithuania.
With Lithuanians unwilling to enter into an alliance with Poland, and wishing to avoid a full-out conflict and international condemnation, Poland staged a fake rebellion by Polish army units (under command by gen. Lucjan Żeligowski) in the Wilno area, which allowed the Polish army to take control of the city in 9 October 1920. The fighting ended this month.
Aftermath
Despite Poland's claim to Wilno, the League of Nations chose to ask Poland to withdraw. Poland didn't comply with the request. Theoretically, British and French troops could have been asked to enforce the League’s decision. France, however, did not wish to antagonize Poland, which was seen as a possible ally in a future war against Germany, and Britain was not prepared to act alone. Thus the Poles were able to keep Wilno, where a puppet goverment Komisja Rządząca Litwy Środkowej of Central Lithuania was formed. A plebiscite was carried out and the Wilno Diet has voted on 20 February 1922 for incorporation into Poland as the capital of the Wilno Voivodship (for more inofrmation, see Vilnius region).
The League of Nations Conference of Ambassadors accepted the status quo in 1923, yet the Wilno region remained a disputed territory between Poland and Lithuania (the latter state still treated Vilnius as its constitutional capital). Lithuanian authorities never accepted the fact that Vilnians elected to be part of Poland. It wasn't understood by Poles, because Poles and Jews made up a majority in city of Vilnius itself; Lithuanians there were just a minority (of less than 3% after WW1, and less than 1% later in 30s). However, in parts of Vilnius region (which, along with Vilnius was taken by Poland), Lithuanians made a majority, and in fact no nationality made up a majority in Vilnius region: there were areas with Polish, Belarussian and Lithuanian majorities, while in some cities there was Jewish majority, and in other cities - clear Jewish minorities. Therefore Lithuania claimed that it also has a right to have a multinational territory, same as Poland, and because this territory was never part of Poland, but it, as well as city of Vilnius, always was historical part of Lithuania, therefore according to this view Lithuania had more rights to the area. Lithuanian constitution continued to mention Vilnius as the capital of the state. All diplomatic relations between Lithuania and Poland were broken and Lithuania declined to accept the Polish authority over the Eastern Lithuania until 1938, when Poland used a international situation of concern over nazi Germany actions and gave ultimatum to Lithuania to renew diplomatic relations and that way de facto recognise Vilnius region as part of Poland.