Its western borders were determined by the victorious Entente and incorporated in the Versailles Treaty. Poland received the Poznan and Pomeranian provinces. The latter contained the Polish Corridor that assured Poland free access to the sea and cut off East Prussia from Germany. A council of League of Nations ambassadors awarded Poland the eastern or coal-rich half of Upper Silesia. Poland's eastern border was determined by war. Poland seized Vilnius from Lithuania and part of Teschen from Czechoslovakia. After Marshal Pilsudski's forces drove the Red Army back from the gates of Warsaw in August 1920, Poland ended up with large sections of Belorussia and the Ukraine under the March 18, 1921 Treaty of Riga. More than 30 percent of the population of post-World War I Poland was ethnically non-Polish, Jews, Lithuanians, Belorussians, Ukrainians, Ruthenians and Germans (Crampton 41).
According to Roberts, both Germany and Russia had "open or suspected designs on territory Poland had acquired from them" (583). It, therefore, became a first axiom of Polish foreign policy to resist revision to the Versailles Treaty and to rely for enforcement of the Versailles settlement on the League of Nations, Poland's 1921 alliances with France and Rumania and, above all, on Poland's own fierce determination to remain independent of pressures from its more populous and powerful neigh