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.
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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.
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.

Natural resources: coal, sulfur, copper, natural gas, silver, lead, salt, amber, arable land.
Ports and harbors: Gdansk, Gdynia, Gliwice, Kolobrzeg, Szczecin, Swinoujscie, Ustka, Warsaw, Wroclaw.
Most of the country is a plain with no natural boundaries except the Carpathian Mountains in the south and the Oder and Neisse rivers in the west.

Its aggressive neighbors and no natural boundaries and some valued natural resources along with some ports made it attractive and vulnerable. Its government at times was not strong enough to resist.