 Stalin 's reputation as a brutal dictator contributed both to the Nazis ' justification of their assault and their faith in success ; many competent and experienced military officers were killed in the Great Purge of the 1930s , leaving the Red Army with a relatively inexperienced leadership compared to that of their German counterparts . The Nazis often emphasized the Soviet regime 's brutality when targeting the Slavs with propaganda . They also claimed that the Red Army was preparing to attack the Germans , and their own invasion was thus presented as a pre @-@ emptive strike . 
