There is no good time for an economy to face an energy- and food-price shock, but some moments are worse than others. And for the United States today – with its strained credit market, stretched equity-price valuations, unsustainable public finances, and inflationary import-tariff policy – now is about as bad as it gets.

The precise magnitude of the shock the US will face as a result of its war of choice in Iran is difficult to predict, given the array of factors at play. But Iran’s effective closure of the Strait of Hormuz, through which 20% of the world’s oil and 20% to 30% of it...