sentences:  Its population also grew between World War I and World War II, reaching a population of 11,000 at its highest point during this time.Along with the rest of industrial Upper Silesia Tychy was occupied by Nazi Germany forces after the invasion of Poland and annexed into the Third Reich, while many of its inhabitants who were not expelled or exterminated were forced to change their nationality to German in order to comply with the racist policies of Nazi Germany.Mass arrests and executions of Polish activists and former Polish insurgents of 1919–1921 were carried out in the first days of the occupation in September 1939.