Mutually Assured Destruction theory?
Yep. He kept a safe in his office and only he had the combination. Speculation was that it obtained dirt on powerful people. It’s why president after president continued to reappoint him FBI Director.
By the way, when you listen to the LBJ tapes of him and Johnson, it sounds like he’s the one in charge. LBJ was afraid of him. Johnson knew Hoover was the one person who could bring him down.
Hoover loved America. We could use an FBI director like that today.