The sultanate of Oman often gets tagged with the cliche of "sleepy" – in part because it has chosen to sit out nearly every major Middle Eastern war since gaining independence in 1971. Bolstered by its reputation for neutrality, it has become an effective diplomatic go-between for bigger powers, including the United States, Iran and Saudi Arabia, allowing it to become friendly to all and foe to none. But all that might be about to change: An assertive Saudi Arabia, under the direction of Crown Prince Mohammed bin Salman, and a resentful Abu Dhabi, led by ambitious Crown Prince Mohammed bin Zayed, sense a chance to reel in Oman and force Muscat to adopt policies that align more closely with their own. Benefiting from a high point in relations with Washington, Riyadh and Abu Dhabi have a window of opportunity to challenge Oman's neutrality. But to succeed, the Gulf giants'...