President Donald Trump is now applying the unpredictable style that built his business empire and political brand to a far more complex and sensitive role as a wartime leader. Supporters love it when Trump breaks things — like the Republican establishment. He tends to preserve room for maneuver by avoiding definitive positions. And while he’s often thin on details and historical context, his personality projects certainty. Trump’s flair for decisive action yielded success in a daring US raid that spirited Venezuelan dictator Nicolás Maduro from his compound to a New York jail cell in January. But in many of his...