Why did Czechoslovakia and Poland dispute control of Teschen?

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Czechoslovak history Saints Cyril and Methodius, mural by Zahari Zograf, 1848; in the Troyan Monastery, Bulgaria.

The dispute over Teschen between Czechoslovakia and Poland arose following the collapse of Austria-Hungary after World War I, primarily because the region was one of the most industrialized and resource-rich areas of the former empire.1

The two nations based their competing claims on different justifications:

Poland’s claim: Based on demographics, as the prewar population of the region was approximately 55 percent Polish.1
Czechoslovakia’s claim: Based on historical arguments regarding the region's long-standing ties to the Bohemian crown.1

The conflict escalated into violence in January 1919 when Czech forces occupied a significant portion of the territory. The dispute was formally resolved in July 1920 by the Allies’ Conference of Ambassadors, which divided the region along the Olše River. Czechoslovakia received the larger portion, including the coal-mining basin and a vital railroad line, while Poland was granted the eastern districts and the city of Teschen. This division left Poland dissatisfied and created strained relations between the two countries for nearly two decades.1