A consummate G-man, J. Edgar Hoover led the FBI for four decades, becoming one of the most influential figures of the 20th century. Demanding rigor, loyalty and stealth from his subordinates, he worked closely with presidents of both parties, but his own views were steeped in conservative ideas on religion, race and anticommunism. As new details emerge about the present-day investigation of Donald Trump, we revisit a conversation with historian Beverly Gage — about Hoover’s imprint on the agency’s culture.