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[Sebatianos]

Stalin had a deal with Hitler so nazis trained their pilots in Soviet Union before the begining of the second world war (even before the Molotov-Ribentrop packt). In that time the Gestapo planted some incriminating documents and some of the pilotes testified that they got top secret information about Soviet military. Stalin led an investigation that most of the army high command was againt, because the charges were ridiculos, so they became the prime suspects.
That's one of the things Stalin regreted most. That's also why this was the last great purge among experts in a certain feild in Soviet Union.
Those new generals were loyal to Stalin - but so were the mases. They truly believed he was their great leader who'll bring them the better world and that he was their closest friend (that's what Tovariš - Commrade means). They were loyal to him because he represented their homeland and the party that saved them from the opression of the feudal monarchs, not because they feared him (you could also say they were brainwashed to be loyal to him).