After President Trump fired former Army general Michael Flynn as national security adviser in February 2017, hope reigned that his replacement, Army Lieutenant General H.R. McMaster, would bring order and professionalism to the vital office that Flynn—who has since pleaded guilty to lying to the FBI about his activities as national security adviser designate—had abused. To generate sound policy, the interagency process led by the national security adviser requires the collegial consideration of a generous range of official viewpoints and perspectives.

Flynn had been unlikely to foster that kind of open conversation. He was a shrill Islamophobe and right-wing ideologue who tolerated no disagreement and recruited acolytes he had groomed in previous active-service positions. Under Flynn’s supervision, the National Security Council would have become a crude vehicle for a far-right agenda. This was in evidence almost immediately, when Trump’s executive order banning citizens of seven predominantly Muslim countries from traveling to the US was issued without being vetted by the State, Defense, or Homeland Security Departments.

As an active-duty soldier, McMaster probably felt compelled to accept the job out of deference to the commander-in-chief—whoever he or she was. His rationale—or at least his rationalization—was likely that the position would best be filled by a warrior-scholar with the spine and rectitude to protect the country against Trump’s rash leadership. No doubt he also found irresistible the opportunity to advance from something of a military backwater—he was then deputy commander of the Army’s Training and Doctrine Command and director of its Army Capabilities Integration Center—to one of the most important jobs in the executive branch.

McMaster had performed brilliantly in the first Gulf War, earning a Silver Star for gallantry in leading a tank assault that destroyed a much larger Iraqi force, and he became the exemplar of US counterinsurgency prowess in pacifying Tal Afar early in the post–September 11 US occupation of Iraq. Between those two wars, he earned a Ph.D. in American history. His dissertation became a widely acclaimed book on civil–military relations, Dereliction of Duty (1997). Its main thesis was that during the Vietnam War the Joint Chiefs of Staff became politicized, deferring to senior civilian officials in the Johnson administration who crafted the policy of “graduated pressure” on North Vietnam.

That policy called for systematically inducing Hanoi to stop supporting the Vietcong through calibrated US military operations. The intent was in part to reduce the US public’s sense of nuclear dread by demonstrating precise American crisis management and military control in Vietnam. McMaster argued that the policy, as applied, failed to present a unified strategy for winning the war. This was hardly a new argument, and remains vulnerable to dispute. But McMaster made a bold and passionate case, asserting that the Vietnam War “was lost in Washington, D.C….even before the first American units were deployed.”1

Enlightened realists tend to make the best national security advisers, and McMaster…