Not every national security adviser needs to shout; some of the most effective have been quiet masters at pulling bureaucratic levers. The problem is, Mr. O’Brien doesn’t seem like much of a lever puller — and in any case there are far fewer levers to pull. The National Security Council has atrophied under Mr. Trump, and especially during Mr. Bolton’s tenure.

Instead, Mr. O’Brien’s appointment is likely to mean that Secretary of State Mike Pompeo will continue to be the president’s chief foreign policy adviser, a role he usurped from Mr. Bolton. Unfortunately, Secretary Pompeo has succeeded precisely because he seems to have few if any principles that he won’t suppress for the sake of holding and wielding executive power. The Ed McMahon of American foreign policy, he is smart enough to be in on the Trump joke, doesn’t care that it’s not funny and laughs anyway.

Some have suggested that Mr. Trump and Mr. Pompeo have formed a foreign-policy partnership comparable to that of Richard Nixon and Henry Kissinger , and that, therefore, we shouldn’t worry about who sits atop the National Security Council. But Mr. Kissinger is an elite scholar, a powerful strategic mind and a policy entrepreneur who led and guided the Nixon administration’s foreign policy. Mr. Pompeo is a non-expert who sees his role as catering to Mr. Trump’s whims and carrying out his ideas.

Mr. Trump most likely hopes that Mr. O’Brien will prove to be not an irrepressible ideologue, a foreign-policy entrepreneur or a consensus-building process expert, but rather a functionary who will provide an official veneer for the president’s often outlandish views and moves.

This would mean the continued decay of the council-led interagency process. Thus unimpeded by contrary voices, Mr. Trump could continue with his tactic of the attempted grand bargain — with China, North Korea, even Iran in certain circumstances that now seem remote — which in turn would be put into motion by blunt instruments like tariffs, initiatives announced by tweet and military bluster and threats.