Zaolzie was the area disputed between Poland and Czechoslovakia West of Cieszyń. In October 1918, the area was divided between Poland and Czechoslovakia by two local self-government councils (Czech Narodni Vybor pro Slezsko and Polish Rada Narodowa). The majority of the area was taken over by Polish local authorities. In 1919, both councils were swallowed by the central governments in Prague and Warsaw. The earlier was not satisfied with the compromise and in 1920 started an offensive while Poland was nearly overrun in course of the Polish-bolshevik war. 66% of the area was then annexed by Czechoslovakia. In October 1938, the area was re-annexed by Poland in accord with the Munich Agreement, but in World War II it was annexed by Germany. After 1945, the border returned to the line of 1920. Nowadays Zaolzie is a part of the Czech Republic.
Border conflicts between Poland and Czechoslovakia
History of Cieszyń and Tešín