[1][2] Between 1942 and 1943, during the German invasion of the Soviet Union, Isselhorst was an Einsatzkommando leader, tasked with the murdering of Jews in what is now Belarus and the Baltic States.

Isselhorst received his doctoral degree in law in 1931, once more returned to Düsseldorf, and joined the Nazi Party in August 1932.

From February 1936 to April 1938 he was head of the Gestapo in Cologne, after which he was transferred to Klagenfurt, Austria, which had recently been annexed by Nazi Germany.

[1][2] From January to October 1942, Isselhorst was transferred to the Reichskommissariat Ostland in occupied Belarus, where he headed a department of the Sicherheitspolizei (SiPo; Security Police).

In September and October 1942 he led Einsatzkommando 8, a sub-group of the mobile killing unit known as Einsatzgruppe B, which was tasked with murdering Jews.

[1][2] Isselhorst returned to Germany in October 1943 and his old role in Munich before being transferred to Strasbourg in December, where he headed the SiPo there.

[3] In January 1945 Isselhorst was transferred once more, now to the Reich Security Main Office (RSHA) in Berlin, where he remained until April.