Great (north) Poland was founded in 966 by Mieszko I, who belonged to the Piast dynasty. The tribes of southern Poland then formed Little Poland. In 1047, both Great Poland and Little Poland united under the rule of Casimir I the Restorer. Poland merged with Lithuania by royal marriage in 1386. The Polish-Lithuanian state reached the peak of its power between the 14th and 16th centuries, scoring military successes against the (Germanic) Knights of the Teutonic Order, the Russians, and the Ottoman Turks.
Lack of a strong monarchy enabled Russia, Prussia, and Austria to carry out a first partition of the country in 1772, a second in 1792, and a third in 1795. For more than a century thereafter, there was no Polish state, just Austrian, Prussian, and Russian sectors, but the Poles never ceased their efforts to regain their independence. The Polish people revolted against foreign dominance throughout the 19th century. Poland was formally reconstituted in Nov. 1918, with Marshal Josef Pilsudski as chief of state. In 1919, Ignace Paderewski, the famous pianist and patriot, became the first prime minister. In 1926, Pilsudski seized complete power in a coup and ruled dictatorially until his death on May 12, 1935.
Despite a ten-year nonaggression pact signed in 1934, Hitler attacked Poland on Sept. 1, 1939. Soviet troops invaded from the east on Sept. 17, and on Sept. 28, a German-Soviet agreement divided Poland between the USSR and Germany. Wladyslaw Raczkiewicz formed a government-in-exile in France, which moved to London after France's defeat in 1940. All of Poland was occupied by Germany after the Nazi attack on the USSR in June 1941. Nazi Germany's occupation policy in Poland was designed to eradicate Polish culture through mass executions and to exterminate the country's large Jewish minority.
The Polish government-in-exile was replaced with the Communist-dominated Polish Committee of National Liberation by the Soviet Union in 1944. Moving to Lublin after that city's liberation, it proclaimed itself the Provisional Government of Poland. Some former members of the Polish government in London joined with the Lublin government to form the Polish Government of National Unity, which Britain and the U.S. recognized. On Aug. 2, 1945, in Berlin, President Harry S. Truman, Joseph Stalin, and Prime Minister Clement Attlee of Britain established a new de facto western frontier for Poland along the Oder and Neisse rivers. (The border was finally agreed to by West Germany in a nonaggression pact signed on Dec. 7, 1970.) On Aug. 16, 1945, the USSR and Poland signed a treaty delimiting the Soviet-Polish border. Under these agreements, Poland was shifted westward. In the east, it lost 69,860 sq mi (180,934 sq km); in the west, it gained (subject to final peace conference approval) 38,986 sq mi (100,973 sq km).