As his first week in office amply demonstrated, Mr. Trump has no grounding in national security decision making, no sophistication in governance and little apparent grasp of what it takes to lead a great diverse nation. He needs to hear from experienced officials, like General Dunford. But Mr. Bannon has positioned himself, along with Mr. Trump’s son-in-law, Jared Kushner, as the president’s most trusted aide, shutting out other voices that might offer alternative views. He is now reportedly eclipsing the national security adviser, retired Lt. Gen. Michael Flynn.

While Mr. Trump long ago embraced Mr. Bannon’s politics, he would be wise to reconsider allowing him to run his White House, particularly after the fiasco over the weekend of the risible Muslim ban. Mr. Bannon helped push that order through without consulting Mr. Trump’s own experts at the Department of Homeland Security or even seeking deliberation by the N.S.C. itself. The administration’s subsequent modifications, the courtroom reversals and the international furor have made the president look not bold and decisive but simply incompetent.

As a candidate, Mr. Trump was immensely gratified by the applause at his rallies for Mr. Bannon’s jingoism. Yet now casually weaponized in executive orders, those same ideas are alienating American allies and damaging the presidency.