"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 German invasion of Poland in 1939, the area became a part of Nazi Germany until 1945. After the war, the 1920 borders were restored."
https://en.wikipedia.org/wiki/Zaolzie
"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 German invasion of Poland in 1939, the area became a part of Nazi Germany until 1945. After the war, the 1920 borders were restored."