Eastern Silesia

Eastern Silesia was a region between newly-created countries Czechoslovakia and Poland at the close of World War I. With no definite border set, ethnic Czechs and and ethnic Poles in the region worked to have the region annexed to one country or the other. In January 1919, Czech troops invaded and occupied part of Eastern Silesia.

A plebiscite was set for 1920, publicized by overprinted Czech and Polish stamps. Czech overprints were only used in Czech-majority towns or occupied areas, Polish overprints were only used in Polish-majority towns. Meanwhile, regular Czech and Polish stamps plus old Austrian stamps were also still used.

Czech issues
 File:E Silesia 20h.jpg|20h perfed. File:E Silesia 500 imperf.jpg|500h imperf. File:E Silesia news.jpg|6h newspaper stamp. File:E Silesia due.jpg|20h postage due. 
 * In 1920, Czech definitives were overprinted SO / 1920. These exist imperf and various values exist in different perfs.
 * Newspaper, special delivery and postage due stamps were similarly overprinted.
 * Overprints on Masaryk commemoratives were issued but did not arrive in time to be used.

Polish issues
 File:E Silesia 15fen.jpg|15f value. File:E Silesia 2.50k.jpg|2.50k value. File:E Silesia Gorny Slask 1.webp|10f insurgent issue imperf. 
 * In 1920, Polish definitives were overprinted S.O. / 1920. (fenigów values) or S.O. 1920. (korona values).
 * In addition, in 1921, ethnic Polish insurgents issued their own stamps inscribed GÓRNY ŚLASK, perf and imperf.

A peace conference was held requiring Czech forces to withdraw from some parts of Eastern Silesia. By August 1920, the borders were set and the plebiscite was never held.