Complex sentence: The Jäger Report was written by Karl Jäger, a commander of one of the "Einsatzgruppen" (special task forces) which were in charge of liquidating Jews, communist leaders, partisans and others in the Soviet Union.
Split into simpler sentences: The Jäger Report was written by Karl Jäger, an "Einsatzgruppen" commander. Einsatzgruppen were special task forces in charge of liquidating Jews, communist leaders, partisans and others within the Soviet Union.