The tensions festering in the Saudi royal family became clear in September, when Joseph Westphal, the U.S. ambassador to Riyadh, flew to Jeddah to meet Crown Prince Mohammed bin Nayef, nominally the heir to the throne. But when he arrived, he was told that the deputy crown prince, a brash 30-year-old named Mohammed bin Salman, wanted to see him urgently.

The ambassador was redirected. The United States and the crown prince swallowed their embarrassment.

Palace intrigue is a staple of monarchies, but it is impossible to overstate how out of character such a generational power play was for the desert kingdom. Robert Lacey, in his classic 1981 book, “The Kingdom,” described the tradition of deference that has held the Saudi royal family together through feast and famine: “Deference to elders is one of the Al-Saud’s inviolable ground rules, the best corset they know to discipline the outward thrust of so many assembled appetites.”

Not anymore: Starting in January 2015 with the accession of King Salman, Saudi Arabia has been shaken by the bold reform campaign of his son, known at home and abroad by his initials, “MBS.” By outmaneuvering and sometimes defying his elders, the young deputy crown prince has turned the politics of this conservative, sometimes sclerotic monarchy upside down.

MBS is the kind of prince that Machiavelli might conjure. He’s a big, fast-talking young man who dominates a room with the raw, instinctive energy of a natural leader. But his hardball tactics have offended some Saudis — especially his rebuffs of Mohammed bin Nayef, his elder at 56 and his nominal superior.

If “Game of Thrones” were set in the Arabian desert, it might have a plot like what has developed in Saudi Arabia over the past 18 months. Anonymous letters have circulated; whispering campaigns have swirled around the deputy crown prince and his rivals. President Obama has advised his aides to avoid any appearance of taking sides. But the president’s White House meeting on June 17 with MBS, treating him almost like a head of state, may have cast an implicit vote of support for the reformer’s agenda.

How did this Saudi political battle begin? Less than a week after the death of King Abdullah on Jan. 23, 2015, the new King Salman issued decrees that altered the balance of power in the kingdom. He removed two of Abdullah’s sons as governors of Riyadh and Mecca, respectively. Prince Bandar bin Sultan, the wily former ambassador to Washington, was ousted as national security adviser.

Salman’s lightning decrees also installed Mohammed bin Nayef as deputy crown prince behind then-Crown Prince Muqrin. And perhaps more important, Salman’s son was named defense minister and chairman of a new Council of Economic and Development Affairs. MBS had his hands on both the military and economic levers of power.

“All this was planned like clockwork,” says one Saudi who watched the maneuvers from Riyadh. “It was a bloodless coup.”

MBS took a giant step up the ladder in April 2015. A royal decree dumped Muqrin as crown prince; Mohammed bin Nayef moved up to the No. 2 spot; and MBS was installed as No. 3. This change in the official succession plan upset some members of the royal family. Although Muqrin was widely seen as an unsuitable potential king, Saudis worried about the precedent, and the possibility the succession plan might be rejiggered again to install MBS.

A decisive blow came in early September when Salman, at his son’s urging, fired Saad al-Jabri, who for years had been Mohammed bin Nayef’s closest adviser. A U.S. source explains what happened: Jabri was coming to America on a personal visit, and he decided to see his old friend John Brennan, the CIA director. He didn’t report this meeting to Salman, and when the king learned what had happened, Jabri was removed.

Can the fragile balance last, with the crown prince and deputy coexisting under an elderly, ailing king? If Salman should leave the scene, would the crown prince succeed him, as the current succession plan provides? Or would MBS try to jump the queue, with acquiescence from a pliant Allegiance Council, which ratifies succession? Saudis don’t know the answers.

The Obama administration, while careful not to take sides in the palace intrigue, seems to agree that the MBS reform agenda offers a chance for the breakthrough that Saudi Arabia needs. But U.S. officials hope the impulsive and sometimes arrogant young prince doesn’t run so fast that he falls over — and takes the kingdom’s political stability down with him.