The Second Republic's land area was 388,634 km2, making it, in October 1938, the sixth largest country in Europe. After the annexation of Zaolzie from Czechoslovakia, this grew to 389,720 km2. 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 population came from minority groups: 13.9% Ukrainians; 10% Jews; 3.1% Belarusians; 2.3% Germans and 3.4% Czechs, Lithuanians and Russians. At the same time, a significant number of ethnic Poles lived outside the country borders. The Republic endured and expanded, despite a variety of difficulties in the aftermath of World War I, including conflicts with the Ukrainians, Czechoslovakia, Lithuania, Soviet Russia and Soviet Ukraine; the Weimar Republic over Greater Poland, and Upper Silesia; and in spite of increasing hostility from Nazi Germany.
Poland maintained a slow (see: trade embargo) but steady level of 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. By 1939, the Republic had become "one of Europe's major powers".
After more than a century of Partitions between the Austrian, the Prussian, and the Russian imperial powers, Poland re-emerged as a sovereign state at the end of the First World War in Europe in 1917-1918. The victorious Allies of World War I confirmed the rebirth of Poland in the Treaty of Versailles of June 1919. It was one of the great stories of the 1919 Paris Peace Conference. Poland solidified its independence in a series of border wars fought by the newly formed Polish Army from 1918 to 1921. The extent of the eastern half of the interwar territory of Poland was settled diplomatically in 1922 and internationally recognized by the League of Nations.
In the course of World War I (1914-1918), Germany gradually gained overall dominance on the Eastern Front as the Imperial Russian Army fell back. German and Austro-Hungarian armies seized the Russian-ruled part of what became Poland. In a failed attempt to resolve the Polish question as quickly as possible, Berlin set up a German puppet state on 5 November 1916, with a governing Provisional Council of State and (from 15 October 1917) a Regency Council (Rada Regencyjna Królestwa Polskiego). The Council administered the country under German auspices (see also Mitteleuropa), pending the election of a king. A month before Germany surrendered on 11 November 1918 and the war ended, the Regency Council had dissolved the Council of State, and announced its intention to restore Polish independence (7 October 1918). With the notable exception of the Marxist-oriented Social Democratic Party of the Kingdom of Poland and Lithuania (SDKPiL), most Polish political parties supported this move. On 23 October the Regency Council appointed a new government under Józef Świeżyński and began conscription into the Polish Army.
In 1918–1919, over 100 workers' councils sprang up on Polish territories; on 5 November 1918, in Lublin, the first Soviet of Delegates was established. On 6 November socialists proclaimed the Republic of Tarnobrzeg at Tarnobrzeg in Austrian Galicia. The same day the Socialist, Ignacy Daszyński, set up a Provisional People's Government of the Republic of Poland (Tymczasowy Rząd Ludowy Republiki Polskiej) in Lublin. On Sunday, 10 November at 7 a.m., Józef Piłsudski, newly freed from 16 months in a German prison in Magdeburg, returned by train to Warsaw. Piłsudski, together with Colonel Kazimierz Sosnkowski, was greeted at Warsaw's railway station by Regent Zdzisław Lubomirski and by Colonel Adam Koc. Next day, due to his popularity and support from most political parties, the Regency Council appointed Piłsudski as Commander in Chief of the Polish Armed Forces. On 14 November, the Council dissolved itself and transferred all its authority to Piłsudski as Chief of State (Naczelnik Państwa). After consultation with Piłsudski, Daszyński's government dissolved itself and a new government formed under Jędrzej Moraczewski. In 1918 Italy became the first country in Europe to recognise Poland's renewed sovereignty.