If anything, Mr. Trump’s civilizational framework makes more sense of America’s forever war in the Middle East than did Barack Obama’s language of surgical strikes. As much as Mr. Obama liked to parse the verbiage and conduct of the “war on terror,” the fact remained that he prosecuted it aggressively and widened it geographically. Mr. Trump has largely continued these policies but suggested that America is in the Middle East to police enemies of civilization. “Lawless savages,” he calls the Islamic State, evoking the worldview of European imperialists and their American admirers like Theodore Roosevelt.

One result was the most surprising action of Mr. Trump’s presidency to date: his strike in April against a Syrian government airfield in retaliation for the use of chemical weapons on civilians. Centrists raved that Mr. Trump might at long last be enforcing universal norms of the so-called liberal international order. But the president implied something else in his public justification: retaliation for a “very barbaric attack.”

In the process, he established another attribute of those who act in the name of civilized humanity — the right to decide, alone, who and what lies beyond the pale.