Zaolzie [zaˈɔlʑɛ] (listen) is the Polish name for an area now in the Czech Republic which was disputed between interwar Poland and Czechoslovakia. The name means "lands beyond the Olza River"; it is also called Śląsk zaolziański, meaning "trans-Olza Silesia". Equivalent terms in other languages include Zaolší (Zaolží) in Czech and Olsa-Gebiet in German. The Zaolzie region was created in 1920, when Cieszyn Silesia was divided between Czechoslovakia and Poland. Zaolzie forms the eastern part of the Czech portion of Cieszyn Silesia. The division did not satisfy any side, and persisting conflict over the region led to its annexation by Poland in October 1938, following the Munich Agreement. After the invasion of Poland in 1939, the area became a part of Nazi Germany until 1945. After the war, the 1920 borders were restored.

Historically, the largest specified ethnic group inhabiting this area were Poles.[1] Under Austrian rule, Cieszyn Silesia was initially divided into three (Bielitz, Friedek and Teschen), and later into four districts (plus Freistadt). One of them, Frýdek, had a mostly Czech population, the other three were mostly inhabited by Poles.[2][3] During the 19th century the number of ethnic Germans grew. After declining at the end of the 19th century,[4] at the beginning of the 20th century and later from 1920 to 1938 the Czech population grew significantly (mainly as a result of immigration and the assimilation of locals) and Poles became a minority, which they are to this day. Another significant ethnic group were the Jews, but almost the entire Jewish population was murdered during World War II by Nazi Germany.

In addition to the Polish, Czech and German national orientations there was another group living in the area, the Ślązakowcy, who claimed a distinct Silesian national identity. This group enjoyed popular support throughout the whole of Cieszyn Silesia although its strongest supporters were among the Protestants in eastern part of the Cieszyn Silesia (now part of Poland) and not in Zaolzie itself.[5]