Abdullah ibn Abdilazīz, who served from 2005 to today as the  Sixth King of Saudi Arabia from 2005 until today, has died at the age of 90, and will be succeeded per tradition by his half-brother Crown Prince Salman:

King Abdullah of Saudi Arabia, who came to the throne in old age and earned a reputation as a cautious reformer even as the Arab Spring revolts toppled heads of state and Islamic State militants threatened the Muslim establishment that he represented, died on Friday, Saudi officials. He was 90.

The cause was unknown. He had been in a hospital since December and placed on a respirator.

Succession was swift. Abdullah’s brother and crown prince, Salman, in a statement attributed to him on Saudi state television, announced the king’s death and that he had assumed the throne.

The royal court, quoted by the official Saudi Press Agency, said the king had a lung infection when he was admitted on Dec. 31 to the Riyadh hospitalnamed for his father, Abdul Aziz, who through conquest and multiple marriages forged the desert kingdom in the years after the destruction of the Ottoman Empire.

Accidents of birth and geology made Abdullah one of the world’s wealthiest and most powerful men. In control of a fifth of the world’s known petroleum reserves, he traveled to medical appointments abroad in a fleet of jumbo jets, and the changes he wrought in Saudi society were fueled by gushers of oil money.

As king he also bore the title of custodian of Islam’s holiest mosques, in Mecca and Medina, making him one of the faith’s most important figures.

Abdullah had grown accustomed to wielding the levers of power long before his ascension to the throne in August 2005. After his predecessor, King Fahd, a half brother, had a stroke in November 1995, Abdullah, then the crown prince, ruled in the king’s name.

Yet Abdullah spoke as plainly as the Bedouin tribesmen with whom he had been sent to live in his youth. He refused to be called “your majesty” and discouraged commoners from kissing his hand. He shocked the 7,000 or so Saudi princes and princesses by cutting their allowances. He was described as ascetic, or as ascetic as someone in the habit of renting out entire hotels could be.

Abdullah’s reign was a constant effort to balance desert traditions with the demands of the modern world, making him appear at times to be shifting from one to the other.

When popular movements and insurgencies overthrew or threatened long-established Arab rulers from Tunisia to Yemen in 2011, he reacted swiftly.

On his return from three months of treatment for a herniated disk and a blood clot in New York and Morocco, his government spent $130 billion to build 500,000 units of low-income housing, to bolster the salaries of government employees and to ensure the loyalty of religious organizations.

He also created a Facebook page, where citizens were invited to present their grievances directly to him, although it was not known how many entries actually reached him.

But in at least two telephone calls he castigated President Obama for encouraging democracy in the Middle East, saying it was dangerous. And he showed no tolerance in his country for the sort of dissent unfolding elsewhere.

The grand mufti, the kingdom’s highest religious official, proclaimed that Islam forbade street protests. Scores of protesters who failed to heed that message were arrested in the chiefly Shiite eastern provinces. A new law imposed crippling fines for offenses, like threatening national security, that could be broadly interpreted.

Reaching beyond his borders, Abdullah sent tanks to help quell an uprising in neighboring Bahrain.

Still, Abdullah became, in some ways, a force of moderation. He contested Al Qaeda’s militant interpretations of the faith as justifying, even compelling, terrorist acts. He ordered that textbooks be purged of their most extreme language and sent 900 imams to re-education sessions. He had hundreds of militants arrested and some beheaded.

But he was also mindful that his family had, since the 18th century, derived its authority from an alliance with the strict Wahhabi sect of Sunni Islam. He accordingly made only modest changes to the kingdom’s conservative clerical establishment. When Islamic State forces conquered vast stretches of Syria and Iraq, imposing a creed linked to Saudi Arabia’s own, the kingdom was slow to respond.

On one occasion, however, Abdullah chastised senior clerics for not speaking out more forcibly against the jihadists, and he eventually sent Saudi pilots to participate in an American-led campaign against the Islamic State.

Abdullah’s Saudi Arabia had hurtled from tribal pastoralism to advanced capitalism in little more than a generation. The fundamentalist clerics who gave the family legitimacy remained a powerful force. Women who appeared in public without the required covering risked arrest or a beating from the religious police.

Abdullah did make changes that were seen as important in the Saudi context. He allowed women to work as supermarket cashiers and appointed a woman as a deputy minister. At the $12.5 billion research university he built and named for himself, women study beside men.

However, he did not fulfill a promise made to Barbara Walters of ABC News in his first televised interview as king in October 2005: that he would allow women to drive, a hugely contentious issue in Saudi Arabia.

Although he ordered the kingdom’s first elections for municipal councils in 2005, a promised second election, in October 2009, in which women would vote, was postponed until September 2011. Then in March of that year, the Ministry of Municipal and Rural Affairs announced that the question of women voting would be put off indefinitely “because of the kingdom’s social customs.”

Abdullah’s greatest legacy, however, may prove to be a scholarship program that sent tens of thousands of young Saudi men and women abroad to study at Western universities and colleges. It has been suggested that the changes long resisted by conservative forces — resistance that even a king could not overcome — would one day come about as those young men and women rose in the government, industry and academia.

Perhaps Abdullah’s most daunting challenge arrived in the wake of the Sept. 11, 2001, terrorist attacks, with the revelation that 15 of the 19 hijackers were Saudis. The royal family at first railed at what it called a vicious public relations campaign against the kingdom, then ruthlessly suppressed known militants — not least because the monarchy itself was a main target of Al Qaeda.

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Abdullah bin Abdul Aziz al-Saud was born in Riyadh in 1924 into a vast, complicated family. His father, Abdul Aziz, had many as 22 wives.

Abdul Aziz, whose ancestors founded a precursor to the present Saudi state in 1744, chose his wives partly to secure alliances with other Arabian tribes. Abdullah’s mother, Fahda bint Asi al-Shuraim, was a daughter of the chief of the Shammar, whose influence extended into Syria, Iraq and Jordan.

Abdullah was Fahda’s only son. She also had two daughters.

King Abdul Aziz was not an indulgent father to his dozens of sons. He was quoted as saying, “I train my own children to walk barefoot, to rise two hours before dawn, to eat but little, to ride horses bareback.”

When the young Abdullah once neglected to offer his seat to a guest, Abdul Aziz sentenced him to three days in prison.

Abdullah, who overcame a stutter, was educated in religion, Arab literature and science by Islamic scholars at the royal court. From the Bedouin nomads, he learned traditional ways, including horsemanship and desert warfare. In 1962, he was appointed commander of the National Guard, which draws recruits from the Bedouin tribes, protects the king and acts as a counterweight to the army.

Four of Abdullah’s half brothers preceded him to the throne.

King Khalid appointed Abdullah as second deputy prime minister in 1975. In 1982, Fahd, Khalid’s successor, named him deputy prime minister and crown prince.

After Fahd’s stroke, Abdullah ran the government at first as regent. Political pressures later forced the removal of the regent title, but Abdullah remained the effective decision-maker until assuming the throne in 2005. He refused to sign any official papers with his own name as long as his stricken brother lived. Fahd died on Aug. 1, 2005.

One of King Abdullah’s first official acts was to pardon two Libyans accused of plotting to kill him, a result of Egypt’s engineering a reconciliation between the two nations. He also pardoned three Saudi academics who were in prison for urging the kingdom to adopt a constitutional monarchy

He went on to establish job-training programs to help ease severe unemployment among educated young Saudis, to develop long-wasted natural gas as a commodity that could be exported, and to bring Saudi Arabia into the World Trade Organization. He became the first Saudi head of state to meet a pope, Benedict XVI, in 2007.

Although he reaffirmed his kingdom’s longstanding alliance with the United States, tensions arose with events. Abdullah refused, for instance, to permit American bases on Saudi territory for the invasion of Iraq in 2003, something he had allowed in the first Gulf War.

The White House has released a statement on King Abdullah’s death:

t is with deep respect that I express my personal condolences and the sympathies of the American people to the family of King Abdullah bin Abdulaziz and to the people of Saudi Arabia.

King Abdullah’s life spanned from before the birth of modern Saudi Arabia through its emergence as a critical force within the global economy and a leader among Arab and Islamic nations.  He took bold steps in advancing the Arab Peace Initiative, an endeavor that will outlive him as an enduring contribution to the search for peace in the region.  At home, King Abdullah’s vision was dedicated to the education of his people and to greater engagement with the world.

As our countries worked together to confront many challenges, I always valued King Abdullah’s perspective and appreciated our genuine and warm friendship.  As a leader, he was always candid and had the courage of his convictions.  One of those convictions was his steadfast and passionate belief in the importance of the U.S.-Saudi relationship as a force for stability and security in the Middle East and beyond.  The closeness and strength of the partnership between our two countries is part of King Abdullah’s legacy.

May God grant him peace.

Given the importance of Saudi Arabia to the region and the world, the death of its leader is obviously a significant event. Pursuant to long-standard tradition, there is no real doubt about how it will play out, although the transition may not be as smooth as some in the past both because of the nature of the international situation at the moment and because it puts the Kingdom one step closer to the day when the generation composed of ibn Saud’s grandchildren will begin to assume power:

Saudi Arabia’s King Abdullah bin Adbul Aziz died Thursday evening, setting the stage for a transition of power at a critical moment as the key U.S. ally in the Middle East struggles with falling oil prices and rising Islamist violence.

The monarch, who was most likely 90, was succeeded by his brother, Crown Prince Salman, according to state television. That put the region’s most important Sunni power and America’s closest Arab ally in the hands of a 79-year old who is reportedly in poor health and suffering from dementia.

Salman’s rise to the throne postpones the question of when the Saudi monarchy will turn to the next generation of princes to run their country of 28 million people at a crucial moment in a region mired in crisis.

While observers in Riyadh widely predicted a smooth transition to Salman, his poor health means his rule could be relatively short. Should there be a power struggle to succeed him, it could leave a policy vacuum in the Middle East at a critical time. Saudi Arabia is a key member of the U.S.-led coalition against the Islamic State and a major ally of the collapsed government in neighboring Yemen.

“Having a king with dementia is the last thing they need at this difficult time,” Henderson said. “Yemen is falling apart, ISIS is knocking at the door … this is an extraordinarily dangerous Middle East from a Saudi perspective.”

By Saudi tradition, the crown passes down among the sons of national founder King Adbulaziz bin Saud, who died in 1953. Salman would be the sixth son of Abdulaziz to be king, and few of his remaining brothers — out of at least 35 who were alive when Abdulaziz died — are believed to be healthy or qualified enough to assume the throne.

In an apparent bid to preempt quarrels about succession in the royal family – and also secure the line for his own favored branch of the family — Abdullah last year took the unprecedented step of anointing a deputy heir, Prince Muqrin, 71, his youngest brother, in a move that analysts believed was a sign of Abdullah’s concern over Salman’s health.

Muqrin is said to be smart and is well-liked among ordinary Saudis; he also has good ties with Saudi Arabia’s most important ally, the United States. But the choice sparked fierce opposition from some of the many excluded princes, who complained that Abdullah was defying a tradition that allows each king to name his own heir. Additionally, Muqrin’s mother was a Yemeni concubine, not a Saudi princess, and some in the family reportedly consider his lineage too impure for him to wear the crown.

By Saudi tradition, King Salman would be free to choose his own successor-in-waiting, but it is widely believed here that he would simply elevate Muqrin from deputy to crown prince.

At that point, the Saudi royal family would face a far more complicated puzzle about who would succeed Muqrin, but it would almost certainly be a prince from the next generation, the grandchildren of Abdulaziz. Hundreds of princes belong to that generation of the House of Saud.

At least initially, there doesn’t seem to be any doubt about succession. The Royal Court has named Salman as the new King and he has assumed that role, but that may not be the end of it. If Salman proves not to be up to the task, then things may get a little complicated:

Henderson said there could be far more maneuvering than the royal family will admit. He said some would privately argue that Salman is not of sound enough mind to run the country, and other factions within the family would push their own favorites.

“The trick is always to try and understand their logic and not be too confined by our own logic,” he said. “Their logic is different. They hate the idea of public show of disunity. So they’ll try to cover that up completely.”

Henderson said “Western logic” would suggest that the Saudis would be smarter to pass over Salman in favor of Muqrin or a next-generation king who would be have the health and faculties to lead the country at an increasingly complex and violent time. Saudi borders a part of Iraq where the Islamic State is powerful, and its southern neighbor, Yemen, is in the midst of a power struggle that Saudis believe will strengthen Iran, its regional rival.

How this succession proceeds will be hugely important to watch over the coming days, weeks, and months. At the very least, it’s worth paying attention to.