The Treaty of Versailles (June 28, 1919) established: 1. that Posnania and Pomerania would return to Poland, which thus almost recovered its western borders in 1772; 2. that plebiscites were held, under the control of the main powers, in Upper Silesia, Warmia and Masuria (Allenstein); 3. that Danzig be established as a free city to serve as Poland’s seaport.
With the military operations, conducted by Piłsudski towards the east, the Polish troops had approximately reached the line of the first partition. No provision existed regarding the eastern borders of Poland, which the main powers had reserved the right to determine (art. 87 of the Treaty of Versailles). With the decision of the supreme council of 8 December 1919, the so-called Curzon line was drawn, which included the ethnographically purely Polish territories on which the new state could until then organize its administration: it included only the Poland of Congress, the region of Białystok and western Galicia to Przemyśl. However, there was an explicit reservation of the rights that Poland could assert over the territories located to the east of this line. At the end of December 1919 the Soviet government made the first peace proposals to Poland which it dropped. Towards the end of April 1920, Pilsudski, at the head of his troops, advanced into Ukraine as far as Kiev, where he remained only for a month. The Bolshevik army then took an offensive, which it carried out successfully, pushing the Polish one back to its capital. The situation in Poland became very critical, because the main powers were unable to help her. But at the decisive moment the patriotism of the Poles rose to the rescue. On August 16, Piłsudski began a counter-offensive movement and completely routed the Soviet troops in the memorable battle near Warsaw. With the Peace of Riga (March 18, 1921) Russia renounced all rights on the territories located west of a line,
According to thenailmythology.com, the plebiscites of Warmia and Mazury took place on 11 July 1920 with an outcome contrary to Poland, which therefore obtained only insignificant border adjustments. That of Upper Silesia took place on March 20, 1921: Germany obtained 707,393 votes, Poland 479,365. The entanglement of the two ethnic elements made the division of the territory foreseen by the Treaty of Versailles very difficult. At the beginning of May the Poles, fearing a disadvantageous decision for them, attempted an insurrection, which was suppressed by international troops. With the decision of the conference of ambassadors of 20 October 1921, Poland obtained the districts of Rybnik and Pszczyna (Pless), the part of the district of Ratibor located on the right bank of the Oder is a part of the industrial zone with the city of Katowice (Kattowitz). At the convention in Spa (10 July 1920), at the time of the Bolshevik invasion, Poland renounced the plebiscite for Teschen: the conference of ambassadors divided the territory with the decision of 28 July favoring Czechoslovakia.
As early as the autumn of 1918, Lithuania reclaimed the city and region of Vilna. Poland had to clear it at the time of the Bolshevik invasion: when, having rejected the latter, it was preparing to take it back, France and England ordered it to abstain. Piłsudski instructed General Żeligowski to occupy it, simulating a revolt. A local diet, elected on January 8, 1922, voted for the union of the region to Poland: this decision was ratified by the Polish constituent diet. With the decision of March 15, 1923, the conference of ambassadors fixed the borders of Poland, not yet determined, leaving the Vilna region to it and recognizing the Russian-Polish ones established by the Riga peace.
On March 17, 1921, the constituent diet approved the definitive constitution based on the French one. At the end of 1922 it was necessary to proceed with the election of the first president of the republic. Piłsudski refused the candidacy, both because he found that the powers attributed to the president were too limited, and because he was exasperated by the relentless hostility that his opponents and especially the National Democratic Party showed him. On December 9, however, a friend of Piłsudski was elected, Gabriele Narutowicz, who was assassinated on the 16th. On the 20th, Stanislao Wojciechowski was elected. Piłsudski assumed the office of Chief of Staff of the Army, which he held until May 1923, when the National Democratic Party came to power with the help of the Peasants’ Party, led by Vincenzo Witos. He then waged a bitter struggle against the new cabinet, as well as against the other three and against parliamentarism: his patriotic merits, his honesty brought him wide popularity in the country and especially in the army. On May 13, 1926 he presented himself in front of Warsaw at the head of some regiments and ordered Wojciechowski to give him back power: having the president of the republic rejected the injunction, he attacked the capital and seized it after a bloody struggle. Wojciechowski resigned. On May 31, the diet elected Piłsudski as president of the republic, who refused. On June 1, Piłsudski’s friend Ignatius Mościcki was elected and was reconfirmed in 1933.