He also thought he could them fighting Mafia. He was an overrated crime fighter. I don't blame him for not knowing how large Mafia was, up to Appalachia arrests no one knew for certain. Even after Valachi, many top crime experts said it did not exist. Hoover also saw communism and socialism as bigger thereat, and he may have been right. There were many spies in America, and many communist supporters. The Rosenbergs stole nuclear secrets and gave them to an enemy, which was at time way larger threat to America than Mafia or any other crime groups.