Complex sentence: The Nazis often emphasized the Soviet regime's brutality when targeting the Slavs with propaganda, also claiming that the Red Army was preparing to attack them, and their own invasion was thus presented as a preemptive strike.
Split into simpler sentences: The Nazis often emphasized the Soviet regime's brutality when targeting the Slavs with propaganda. German propaganda claimed the Red Army was preparing to attack them, and their own invasion was thus presented as a preemptive strike.