Few Americans have heard of Qaboos bin Said al-Said, the sultan of Oman. But when he died Friday at 79, the U.S. lost an invaluable interlocutor. In nearly 50 years on the throne, Qaboos time and again helped presidents navigate delicate diplomatic and military challenges.

The sultan kept such a low profile that the adjective “reclusive” was almost obligatory in articles about him. But when President Obama’s efforts to restrict Iran’s nuclear-weapons program stalled in 2012, the interview I had long requested was suddenly...