MUSCAT, Aug. 17 -- When diplomats from Tehran and Muscat agreed in early August on the coordinates of shipping lanes through the Strait of Hormuz, it appeared, briefly, like the geometry of a deal. Inbound ships entering through Iranian waters. Outbound vessels departing through Omani. A map that might thread the needle of mutual face-saving and get oil moving again.

By Monday, the sultanate was being threatened with American bombs.

President Trump, speaking by phone to Fox News journalist Trey Yingst, turned on the country that has quietly served for decades as Washington's back channel to Tehran. "If Oman gets in the way, we'll bomb the shit out of them," Trump said, on a day when a 60-day memorandum of understanding between the Unite...