President Trump called off a military strike against Iran in mid-mission Thursday, and his supporters and even some of his critics are hailing it as an act of restraint and courage. The question for American interests is whether Iran and other adversaries will see it instead as a sign of weakness and indecision.

“We were cocked & loaded to retaliate last night on 3 different sights when I asked, how many will die. 150 people, sir, was the answer from a General. 10 minutes before the strike I stopped it, not proportionate...