Four officials said that striking the three targets would result in about 150 casualties, a number derived from Iranian manning doctrine for these particular facilities, including operators, maintenance personnel and security guards.

How much Mr. Trump was paying attention to that part of the briefing or what he absorbed was not clear in hindsight to some officials. But they said the casualty estimates were included as part of the target package presented to the president…

Still, there continued to be pushback from Pentagon civilians and General Dunford. They argued that killing as many as 150 Iranians did not equate to the shooting down of a drone and could prompt a counterstrike by Iran that would escalate into a broader confrontation…

Mr. Trump seemed to be nursing doubts of his own, partly because of reports that the Iranian commander who shot down the drone had acted on his own, not on specific orders of the national government. Just after the Situation Room meeting, he sat down with Prime Minister Justin Trudeau of Canada, who was visiting, and floated that scenario.