Camera bulbs flashed. Spectators jostled for a better view. All eyes were on the dark-haired director of the Federal Bureau of Investigation, whose integrity had been unquestioned until his search for collusion between Russia and U.S. government officials angered the president. When asked to back off, the director had refused. Now, at a congressional hearing, he was about to declare publicly his independence from the president.

That FBI director was J. Edgar Hoover. The president was Harry S. Truman. And the testimony occurred...