There was a short-lived war scare following Iran’s Sept. 14 attack on Abqaiq, an important Saudi oil-production facility, and a nearby oil field. The scare faded as pundits concluded that underneath the chest-thumping, Donald Trump was more chicken than hawk—that for all his “cocked and loaded” bluster, he is ultimately willing to let Iran get away with mayhem in the Gulf and possibly even willing to cut a new deal with Tehran on its terms.

That’s a misreading of President Trump and his political base. Jacksonian America...