MUSCAT, Aug. 18 -- Shipping traffic through the Strait of Hormuz, the waterway that once carried 130 vessels a day and roughly 20 percent of the world's oil supply, has collapsed to a fraction of normal. Three commercial crossings were recorded on Sunday, down from 19 the previous Monday, a 19.5 percent drop in a single week. Oil prices closed at $90.87 per barrel, up 2.7 percent on the day. And the 60-day memorandum of understanding that was supposed to prevent exactly this kind of deterioration expired Monday without replacement.

What followed was not a fresh negotiating round or a diplomatic briefing. Donald Trump called into Fox News and threatened to bomb Oman.

"If Oman gets in the way, we'll bomb the shit out of them," Trump said,...