The Upper Silesia plebiscite was a plebiscite mandated by the Versailles Treaty and carried out on 20 March 1921 to determine a section of the border between Weimar Germany and Poland. The region was ethnically mixed with both Germans and Poles; according to prewar statistics, ethnic Poles formed 60 percent of the population. Under the previous rule by the German Empire, Poles claimed they had faced discrimination, making them effectively second class citizens. The period of the plebiscite campaign and inter-Allied occupation was marked by violence. There were three Polish uprisings, and German volunteer paramilitary units came to the region as well.