Upper Silesia (Horní Slezsko; Oberschlesien; Latin: Silesia Superior; Górny Śląsk; Silesian: Gůrny Ślůnsk) is the southeastern part of the historical and geographical region of Silesia; Lower Silesia is to the northwest. Throughout its history Upper Silesia has been under the control of Poland, and as part of Holy Roman Empire of Bohemia, Austria, Prussia, and later of unified German Reich. It is currently split between Poland (Opole and Silesian Voivodeships) and the Czech Republic (Czech Silesia, or the Silesian-Moravian Region).
At the time of Svatopluk I and King Arnulf of Carinthia in the ninth century, Silesia was a part of Greater Moravia and after its destruction in the early tenth century it was conquered by Bohemia. A number of earlier inhabitants of Silesia, the Silingi, remained throughout and they concentrated around the Zobten mountain and in a settlement named Niempsch (derived from a Slavic name for Germans).
Upper Silesia was soon conquered by the newly installed dukes of the Polans and for several hundred years was part of Poland. This fell apart and at the renewal of Poland under Casimir the Great, all of Silesia was specifically excluded as non-Polish land. In 1335 it came back under the rule of the Kingdom of Bohemia. Many towns were destroyed by the Mongols at the Battle of Legnica but rebuilt. By the 1300s influx of settlers to Upper Silesia stopped, because of the plague. Latin, Czech and German language were used for towns and cities and only in the 1550s with the Protestant Reformation did records with Polish names also appear. A large number of Silesians became Protestants, when all of Upper Silesia belonged to the Hohenzollerns of Brandenburg-Ansbach. The Roman Catholic Holy Roman Emperors of the Habsburg dynasty reintroduced Catholicism, led by the Jesuits.
Lower Silesia and most of Upper Silesia became part of the Kingdom of Prussia in 1742 during the First Silesian War. A small part remained within the Habsburg-ruled Bohemian Crown as the Duchy of Upper and Lower Silesia, colloquially called Austrian Silesia.
In 1919 after World War I, the eastern part, which had majority of ethnic Poles, came under Polish rule as the Autonomous Silesian Voivodeship, while the mostly German-speaking western part remained part of the German Reich as the Province of Upper Silesia. From 1919-1921 three Silesian Uprisings occurred among the Polish-speaking populace of Upper Silesia; the Battle of Annaberg occurred within the region in 1921. In the Upper Silesia plebiscite a vote of 60 to 40 percent voted against joining to Poland, with clear lines dividing Polish and German communities. As a result the outcome of the vote served as basis for Polish-German border.
After 1945 almost all of Upper Silesia became part of Poland. A majority of the German-speaking population was expelled in accordance with the decision of the victorious Allied powers at their 1945 meeting at Potsdam. This expulsion program also included German speaking inhabitants of Lower Silesia, eastern Pomerania, Danzig, and East Prussia. These German expellees were transported to the present day Germany (including the former East Germany), and they were replaced with Poles, many from former Polish provinces taken over by the USSR in the east. A good many German-speaking Upper Silesians ended up being relocated in Bavaria. A small part of Upper Silesia stayed as part of Czechoslovakia as Czech Silesia.
The expulsions of German-speakers did not totally eliminate the presence of a population that considered itself German. Upper Silesia in 1945 had a considerable number of Roman Catholic mixed bilingual inhabitants that spoke both German and Polish dialects, and their Polish linguistic skills were solid enough for them to be allowed to remain in the area. With the fall of communism and Poland joining the European Union, there were enough of these remaining in Upper Silesia to allow for the recognition of a German minority by the Polish government.
Excerpted from: Virchow RC. Collected Essays on Public Health and Epidemiology. Vol 1. Rather LJ, ed. Boston, Mass: Science History Publications; 1985:204–319.