But by the second decade of this century, the gerontocracy was no longer sustainable. The brothers in line for the throne were dying off before they could sit on it, finally opening the way for one of the last of them, Salman bin Abdulaziz, the father of M.B.S., to take power in January 2015. He was 79 and, by many accounts, would soon show hints of dementia. (The Saudi royal court has denied that King Salman suffers from mental impairment.)

M.B.S. had several older half brothers, including one who had flown as an astronaut on the American space shuttle, but by the time his father ascended the throne, the brash 29-year-old M.B.S. was well established as the favorite. While the others were educated abroad and lived much of their lives outside the kingdom, M.B.S. had stayed close to home and to Salman, the governor of Riyadh.

He “never ran a company that made a mark,” Hubbard writes. “He never acquired military experience. He never studied at a foreign university. He never mastered, or even became functional in, a foreign language. He never spent significant time in the United States, Europe or elsewhere in the West.” Yet suddenly there he was, the rising star in the royal palace.

M.B.S. immediately acquired important portfolios as minister of defense and became the gatekeeper for the king as head of the royal court. He would later brag that in the first 10 days of his father’s rule, “the entire government was restructured.” The pace of disruption was extraordinary and very quickly became dangerous. In March 2015, barely two months after he took over the Defense Ministry, M.B.S. ordered the until then mostly decorative Saudi Air Force to start bombing Yemen, which was in the midst of a civil war.

The operation was supposed to last weeks and intimidate Iran, which has supported one of the warring factions. But the fighting continues to this day, accumulating a grim record of civilian casualties, many of them killed by bombs supplied by the United States to the Saudis. Disease has added to the misery of what has become, according to the United Nations, the world’s worst humanitarian disaster.

Although an older, much more experienced cousin of M.B.S. had been made crown prince and heir apparent, palace insiders could see early on that the cousin would not be around for long. The bond between M.B.S. and his father the king was too close. “Between the onion and the skin there is only the stink,” was an often repeated saying around Riyadh. And by the summer of 2017, M.B.S. had forced his rival out.