Presidents and attorneys general of both parties have been reluctant to use the Espionage Act when secret information has been leaked to the press because they have recognized that it is overbroad.

They have understood, as Moynihan argues in “Secrecy,” that government classifies far too many things as secrets, even as it has often failed to protect information that truly needs to stay secret.

Barack Obama and his Justice Department seem to be of a different mind. They have used the Espionage Act of 1917 six times to bring cases against government officials for leaks to the media six times — twice as many as all their predecessors combined.

“Gradually, over time,” Moynihan writes, “American government became careful about liberties.” Now, suddenly, it seems to be moving in the other direction.