The Polish Parliament establishes Silesian Voivodeship before the Polish-German plebiscite.

The Upper Silesia plebiscite was a plebiscite mandated by the Versailles Treaty and carried out on 20 March 1921 to determine ownership of the province of Upper Silesia between Weimar Germany and Poland. The region was ethnically mixed with both Germans and Poles; according to prewar statistics, ethnic Poles formed 60 percent of the population. Under the previous rule by the German Empire, Poles claimed they had faced discrimination, making them effectively second class citizens. The period of the plebiscite campaign and inter-Allied occupation was marked by violence. There were three Polish uprisings, and German volunteer paramilitary units came to the region as well.

The area was policed by French, British, and Italian troops, and overseen by an Inter-Allied Commission. The Allies planned a partition of the region, but a Polish insurgency took control of over half the area. The Germans responded with volunteer paramilitary units from all over Germany, which fought the Polish units. In the end, after renewed Allied military intervention, the final position of the opposing forces became, roughly, the new border. The decision was handed over to the League of Nations, which confirmed this border, and Poland received roughly one third of the plebiscite zone by area, including the greater part of the industrial region.After the referendum, on 20 October 1921, a conference of ambassadors in Paris decided to divide the region. Consequently, the German-Polish Accord on East Silesia (Geneva Convention), a minority treaty, was concluded on 15 May 1922 which dealt with the constitutional and legal future of Upper Silesia that had partly become Polish territory.

The Second Polish Republic, at the time officially known as the Republic of Poland, was a country in Central and Eastern Europe that existed between 1918 and 1939. The state was established in 1918, in the aftermath of the First World War. The Second Republic ceased to exist in 1939, when Poland was invaded by Nazi Germany, the Soviet Union and the Slovak Republic, marking the beginning of the European theatre of the Second World War.

In 1938, the Second Republic was the sixth largest country in Europe. According to the 1921 census, the number of inhabitants was 27.2 million. By 1939, just before the outbreak of World War II, this had grown to an estimated 35.1 million. Almost a third of the population came from minority groups: 13.9% Ruthenians; 10% Ashkenazi Jews; 3.1% Belarusians; 2.3% Germans and 3.4% Czechs and Lithuanians. At the same time, a significant number of ethnic Poles lived outside the country's borders.

When, after several regional conflicts, the borders of the state were finalised in 1922, Poland's neighbours were Czechoslovakia, Germany, the Free City of Danzig, Lithuania, Latvia, Romania and the Soviet Union. It had access to the Baltic Sea via a short strip of coastline either side of the city of Gdynia, known as the Polish Corridor. Between March and August 1939, Poland also shared a border with the then-Hungarian governorate of Subcarpathia. The political conditions of the Second Republic were heavily influenced by the aftermath of the First World War and conflicts with neighbouring states as well as the emergence of Nazism in Germany.

The Second Republic maintained moderate economic development. The cultural hubs of interwar Poland – Warsaw, Kraków, Poznań, Wilno and Lwów – became major European cities and the sites of internationally acclaimed universities and other institutions of higher education.