The peacemakers did require the successor states to guarantee minority rights, but these treaties soon proved a dead letter.

From The Long Shadow: The Legacies of the Great War in the Twentieth Century by David Reynolds.  Page 14.

The peacemakers did require the successor states to guarantee minority rights, but these treaties soon proved a dead letter. Piłsudski’s Poland was unapologetically racist, while most Serbs regarded Yugoslavia as convenient disguise for Greater Serbia. Even Czechoslovakia, the most tolerant and democratic of these states, did not treat its minorities well. Masaryk, the Czech nationalist son of a Slovak father, was convinced that the cultural and linguistic differences between the two peoples were relatively insignificant. But for three centuries the Czechs had lived under German rule, opening them to Protestantism, the West, and industrialization. The Slovaks, by contrast, had lived even longer under the Hungarians: their Catholic, largely rural society was intertwined economically with Hungary and Ukraine. Such fundamental differences could not easily be bridged, and the Slovak nationalist leader Father Andrej Hlinka was soon clamoring for Slovak autonomy. The German question was handled even more autocratically by Masaryk and his inner circle. They moved quickly to break up the large estates, mostly German-owned, which Czechs applauded as long-overdue reparation for the Habsburg conquest of 1620. Foreign minister Edvard Beneš told a British diplomat bluntly: “Before the war, the Germans were here” (pointing to the ceiling) “and we were there” (pointing to the floor). “Now,” he declared, reversing his gestures, “we are here and they are there.” Land reform, Beneš insisted, was “necessary” to teach the Germans a “lesson.” But it was a lesson the Germans would not accept, as Beneš would learn the hard way in 1938.