President Trump’s Iran policy over the weekend was both erratic and masterful. Doves and isolationists, panicked by what they see as the administration’s inexorable drift toward war, rejoiced when Mr. Trump announced that a military strike had been called back. Hawks criticized him for an Obama-like climb-down, but the announcement of cyberattacks and tightening sanctions helped smooth ruffled feathers.

The result? Mr. Trump more than ever dominates U.S. Iran policy; contending political factions within the administration...