Trump's unpredictability causes great anxiety in Europe, particularly his questioning of the mutual defense promise at the heart of the alliance.

Mr. Trump had been eager to celebrate his success at NATO and revel in the fact that he had conducted an attack that none of his predecessors had dared to launch. His view was backed up by Mark Rutte, the secretary general of the alliance, who wrote Mr. Trump a private message thanking him for his “decisive action” in Iran.