“Abolish the FBI!” New Leftists might have demanded half a century ago—if they thought the idea had the slightest chance of being taken seriously. The bureau, radicals correctly assumed, was bent on dividing and destroying their movements. Undercover agents moled inside antiwar and Black power groups, amassing voluminous files on thousands of activists. Other operatives wiretapped hotel rooms where Martin Luther King Jr. stayed to record him having extramarital sex. Knowledge of the tapes, they hoped, would force the civil rights leader to abandon the cause or even drive him to suicide. FBI Director J. Edgar Hoover had long vowed to protect the nation from “subversives” of all kinds. Thousands of his minions were dispatched to pursue that mission.

After the FBI recovered classified documents from Mar-a-Lago in August, zealots such as Marjorie Taylor Greene turned that bygone whim of the left into a combative hashtag for the right. Since FBI agents began looking into the affairs of Donald J. Trump in 2016, his MAGA apostles have consistently accused the bureau of being the citadel of a “Deep State” that means to rip away the liberties of every patriotic citizen. “Political Hacks and Thugs,” the former president cursed the FBI agents who conducted the search of his resort. Tucker Carlson even accused the bureau of inciting the January 6 assault on the Capitol.

If that ideological turnabout seems ironic, Beverly Gage’s magnificent biography of Hoover, G-Man: J. Edgar Hoover and the Making of the American Century, shows how the institution has always been something of a moving political target. In absorbing detail and lapidary prose, Gage, a historian at Yale, makes clear that Hoover designed his bureau to be a potent tool for advancing the security needs of a growing national government—no matter which president or party controlled it. To be sure, Hoover’s own politics were steadfastly conservative and white supremacist; while in law school at George Washington University, he joined a fraternity drenched in the romance of the Confederacy and remained loyal to its “Southern sentiments” throughout his life. His determination to destroy the Communist Party also made him a darling of the right during the Cold War. But he gladly served the political needs of both Franklin Roosevelt and Lyndon Johnson, those paragons of New Deal liberalism. In gratitude, LBJ, relates Gage, “showered” the FBI director “with Christmas gifts and phone calls, lunch invitations and pleas for elder-statesman wisdom” and even called him “my brother and personal friend.”

Above all, Hoover deserves to be remembered as a primary architect of the modern administrative state, whatever one thinks of its purposes. To learn how he expanded the FBI’s powers and thus his own is to understand something essential about high politics that historians who focus narrowly on public dramas and their outcomes often miss. As Gage observes: “Hoover’s popular image suggests that exercising power is a simple task: press a few buttons, whisper in a few ears, twist a few arms, and presto, the world opens up. The truth is that power does not simply arrive. It has to be created, policy by policy, law by law, step by excruciating step.” Those who master such an incremental approach create structures that will endure and likely prosper, long after an egomaniac like Trump is gone.