Following World War I, Brandt joined the ""Marinebrigade Ehrhardt"" and fought with the Freikorps in the Spartacist uprising of the German Revolution in Berlin, and in the Silesian Uprisings against the Poles and Polish Silesians of Upper Silesia. In 1921 he joined the Nazi Party and was involved in the assassination of the Foreign Minister Walther Rathenau in June 1922. He was sentenced to four years imprisonment in 1925. He was released early and studied medicine from 1926 to 1932 in Kiel, Berlin and Munich.