That would have required a modicum of trust

From The Long Shadow: The Legacies of the Great War in the Twentieth Century by David Reynolds.  Page 12.

The statesmen at Paris were not, therefore, architects of the new Europe—more like firefighters desperately trying to pour water on the flames. Maps and statistics were woefully inadequate, and the competing states dressed up the demographic evidence to their own advantage. As Wilson and his advisers began to realize, poring over their beautiful maps in the elegant Hôtel Murat, neat, clean lines could not be drawn through ethnically mixed regions whose inhabitants were now angrily self-conscious.

One small but revealing example is the small duchy of Teschen—less than nine hundred square miles in area, smaller than Rhode Island, but including part of the Silesian coalfield and a strategic railway junction. After the Habsburg collapse, the duchy became a bone of contention between Poland and Czechoslovakia. On ethnic grounds the Poles had a strong case, but the Czechoslovaks argued that coal from the mines was essential for their industries and that the railway was a vital link between the two halves of their new state. Although Polish-speakers constituted a majority of the population, relations between them and the Czechs, Germans, and Silesians had been reasonably tolerant before 1914, but in January 1919 troops from the rival countries moved in. Fighting ensued, followed by riots among the populace. With tensions too high to allow a plebiscite, the Allies partitioned the duchy in July 1920. The Czech state got most of the coalfield (even though the miners were largely Poles) while the city of Teschen was split in two with the old quarter allocated to Poland and the suburbs, including the railway station, to the Czechoslovaks. According to one frustrated American participant, “The electric light plant goes to the one state, but the gas works to the other, and I do not recall what was to become of the municipal water-works.” These disputes could have been resolved by economic and transit agreements but that would have required a modicum of trust between Poland and Czechoslovakia which simply did not exist after 1918. The Teschen settlement further poisoned relations between Poland and Czechoslovakia, two new states that should have felt a common interest in containing German revanchism.