After World War I, Austrian Silesia was claimed by the new Czechoslovakia. As in the Cieszyn region the Polish-speaking group was predominant. The controversy had to be solved by a plebiscite but finally in 1920 the region was simply divided between both countries. Since the Polish-speaking inhabitants were still the majority in the Czech part, most of it was annexed by Poland in 1938. After World War II however, the 1920 boundary was restored.
For more information, I recommend a visit to Lubor Kunc's page about Eastern Silesia 1918 - 1920
German propaganda overprints: