The U.S. and Israeli strikes against Iran began only hours ago, but there’s already one clear lesson for the rest of the world: If you have a nuclear weapon, you are safe from potential U.S. attack, and if you don’t have a nuclear weapon, you are vulnerable. In 2018, President Trump violated a multilateral nuclear agreement—the Joint Comprehensive Plan of Action with Iran—that was working. Less than a decade later, he has decided with no sound legal or military justification to launch a barrage...