In April 1941, Nazi Germany invaded Yugoslavia and Slovene Styria was annexed to the Third Reich. A policy of violent Germanization was introduced. Public use of Slovene was prohibited, and all Slovene associations were dissolved. Members of all professional and intellectual groups, including many clergymen, were expelled. Between April 1941 and May 1942, around 80,000 Slovenes (almost 15% of the overall population) were expelled from Lower Styria, or resettled to other parts of the Reich. As a reaction, a resistance movement developed. Many areas of Lower Styria witnessed fierce fighting between German troops and Slovene partisan units.