The history of interwar Poland comprises the period from the revival of the independent Polish state in 1918, until the Invasion of Poland from the West by Nazi Germany in 1939 at the onset of World War II, followed by the Soviet Union from the East two weeks later. The two decades of Poland's independence between the world wars are known as the Interbellum.

Poland re-emerged in November 1918 after more than a century of partitions by Austria-Hungary, the German, and the Russian Empires. Its independence was confirmed by the victorious powers through the Treaty of Versailles of June 1919, and most of the territory won in a series of border wars fought from 1918 to 1921. Poland's frontiers were settled in 1922 and internationally recognized in 1923. The Polish political scene was democratic but chaotic until Józef Piłsudski (1867–1935) seized power in May 1926 and democracy ended. The policy of agrarianism led to the redistribution of lands to peasants and the country achieved significant economic growth between 1921 and 1939. A third of the population consisted of minorities—Ukrainians, Jews, Belarusians, Lithuanians and Germans.

The independence of Poland had been successfully promoted to the Allies in Paris by Roman Dmowski and Ignacy Paderewski. U.S. President Woodrow Wilson made the independence of Poland a war goal in his Fourteen Points, and this goal was endorsed by the Allies in spring 1918. As part of the Armistice terms imposed on Germany, all German forces had to stand down in Poland and other occupied areas. So as the war ended, the Germans sent Piłsudski, then under arrest, back to Warsaw. On November 11, 1918, he took control of the puppet government the Germans had set up. Ignacy Daszyński headed a short-lived Polish government in Lublin from November 6 but Piłsudski had overwhelming prestige at this point. Daszyński and the other Polish leaders acknowledged him as head of the army and in effect head of what became the Republic of Poland. Germany, now defeated, followed the terms of the Armistice and withdrew its forces. Jędrzej Moraczewski became the first prime minister (in November 1918) and Dmowski headed the largest party. From its inception, the Republic fought a series of wars to secure its boundaries. The nation was rural and poor; the richest areas were in the former German areas in the west. Industrialization came very slowly, and was promoted in the mid-1930s with the development of the Central Industrial District.

Most Polish leaders of that period wanted to create a larger Polish state; one optimal plan, dating back to the Paris Peace Conference, included the incorporation of East Prussia and the German city of Königsberg being placed in a customs union with Poland. At the same time, the exact boundaries of the former Polish–Lithuanian Commonwealth were not desired, though mentioned as an opening gambit by Roman Dmowski. Much of this land had been controlled by the Russian Empire since the Partitions of Poland and its inhabitants were struggling to create their own states (such as Ukraine, Belarus, and the Baltics: Lithuania, Latvia, Estonia). The Polish leadership did not aim to restore the nation to its 17th-century boundaries. Opinions varied among Polish politicians as to how much of the territory a new, Polish-led state should contain and what form it should take. Józef Piłsudski advocated a democratic, Polish-led federation of independent states — while Roman Dmowski leader of the Endecja movement represented by the National Democratic Party, set his mind on a more compact Poland composed of ethnic Polish or 'polonizable' territories.

To the southwest, Poland and Czechoslovakia contested boundary disputes (see: Trans-Olza). More ominously, an embittered Germany begrudged any territorial loss to its new eastern neighbor. The December 27, 1918 Great Poland Uprising liberated Greater Poland. The 1919 Treaty of Versailles settled the German-Polish borders in the Baltic region. The port city of Danzig (Polish: Gdańsk), with a majority German population and Polish minority was declared a free city independent of Germany, and became a bone of contention for decades. Allied arbitration divided the ethnically mixed and highly coveted industrial and mining district of Upper Silesia between Germany and Poland, with Poland receiving the smaller in size, but a more industrialized eastern section in 1922, after series of three Silesian Uprisings.

The military conflict with the Soviets proved the determinant of Poland's frontiers in the east, a theater rendered chaotic by the repercussions of the Russian revolutions and subsequent civil war. Piłsudski envisioned creating a federation with the rest of Ukraine (led by the Polish-friendly government in Kiev he was to help to install) and Lithuania, thus forming a Central and East European federation called "Intermarium" (Polish: "Międzymorze", literally "area between seas"). Lenin, leader of the new communist government of Russia, saw Poland as the bridge over which communism would pass into the labor class of a disorganized postwar Germany. And the issue was further complicated as some of the disputed regions had assumed various economic and political identities since the partition in the late 18th century while some did not have an ethnically Polish majority in the first place they were still viewed by Poles as their historic regions since they envisioned Poland as a multiethnic state. In the end, the negotiations broke down, sinking Piłsudski's idea of Międzymorze federation; instead, wars like the Polish-Lithuanian War or the Polish-Ukrainian War decided the borders of the region for the next two decades.

The Polish-Soviet war, began in 1919, was the most important of the regional wars. Piłsudski first carried out a major military thrust into Ukraine in 1920 and in May Polish-Ukrainian forces reached Kiev. Just a few weeks later, however, the Polish offensive was met with a Soviet counter-offensive, and Polish forces were forced into a retreat by the Red Army. Poland was driven out of Ukraine and back into the Polish heartland. Most observers at the time marked Poland for extinction and Bolshevization, However at the Battle of Warsaw Piłsudski organized a stunning counterattack that won a famous victory. This "Miracle on the Vistula" became an iconic victory in Polish memory. Piłsudski resumed the offensive, pushing the Red forces east. Eventually, both sides, exhausted, signed a compromise peace treaty at Riga in early 1921 that divided the disputed territories of Belarus and Ukraine between the two combatants. These acquisitions were recognized by the international agreement with the Entente. The treaty gave Poland an eastern border well beyond what the peacemakers in Paris had envisioned and added 4,000,000 Ukrainians, 2,000,000 Jews, and 1,000,000 Belarusians to Poland's minority population.

In Soviet historiography, the Polish-Soviet War was also referred to as a "war against White Poles", with the epithet belopoliaki ("White Poles") smearing the Poles as "counter-revolutionaries" akin to the White Russians. In 1922, in the aftermath of the Polish-Soviet War and Polish-Lithuanian War, Poland officially annexed Central Lithuania following a plebiscite, which was never recognized by Lithuania. The Riga arrangement influenced the fate of the entire region for the years to come. Ukrainians and Belarusians found themselves without a country or province of their own, and some Polish speakers also found themselves within the borders of the Soviet Union. The latter experienced forced collectivistion, state terror, suppression of religion, purges, labor camps and famine. The newly formed Second Polish Republic, one-third of whose citizens were non-ethnic Poles, engaged in promoting Polish identity, culture and language at the expense of the country's ethnic minorities who felt alienated by the process.