The death of Crown Prince Sultan bin Abdul-Aziz, at the age of 80 (some sources say he was six years older), highlights the dangers of a power vacuum in Saudi Arabia, which is responsible for 25% of the world's oil. The kingdom is ruled by a frail gerontocracy, despite its oil wealth and political vulnerability. Sultan was the half-brother of the 87-year-old King Abdullah. The conservative Wahhabi kingdom has survived the Arab spring largely because Abdullah has provided $130bn (£81.5bn) in welfare aid.
As the Arab spring unwound, Sultan's role as Saudi defence and aviation minister was crucial, particularly in Bahrain, whose minority Sunni regime was supported by Saudi tanks in March. That adventure was run by Prince Nayef, the 78-year-old interior minister, who is likely to replace Sultan as crown prince.
Born in the Saudi capital, Riyadh, Sultan was the second of the elite "Sudairi seven", the sons of the kingdom's founder, King Abdul-Aziz Al Saud (commonly known as Ibn Saud) by his wife Hussa bint Ahmad al-Sudairi. Ibn Saud had some 45 recorded sons by as many as 32 wives of whom she was pre- eminent. A dominant mother, she insisted that all of her high-profile sons dine with her once a week. Fahd, the eldest, eventually became king in 1982.
Sultan received an early education in theology, modern culture and diplomacy at the royal court, where he also acquired good English. He became governor of Riyadh in 1947. In that year he oversaw the Arabian American Oil Company's (Aramco's) construction of a rail link between Dammam in the eastern province and Riyadh. He joined the cabinet in 1953, the year that Ibn Saud died and was succeeded as king by his son Saud. Sultan became agriculture minister, helping to settle Saudi Arabia's bedouin on modern farms and was considered hardworking and pugnacious. In 1955 he was appointed communications minister, a post he held until 1962.
That year Sultan became the kingdom's defence and aviation minister. When it became clear to the princes that King Saud was a failure, Sultan pressed for his abdication in favour of the charismatic Faisal, demanding that it should be brought about by force, if necessary. He stepped down in March 1964, and Faisal became king that November. Sultan was considered intelligent with innate worldly wisdom, a man on whom the king might rely.
Sultan oversaw Saudi Arabia's participation in North Yemen's bloody civil war in the 1960s which pitted monarchists against republicans supported by Nasser's Egypt. After the 1963 ceasefire the UN Swedish commander Carl von Horn believed that Sultan, "a volatile and emotional young man", had breached the ceasefire, allowing arms to cross to monarchist rebels against the wishes of King Faisal. Sultan's attempts to support the South Yemen sheikhs after Britain's withdrawal from Aden ended in fiasco when his liberation army based in Najran was cut to pieces by troops in the south inherited from the British colonial authorities.
Faisal was assassinated in 1975, and Khaled became king, with Fahd named crown prince. When Egypt signed the Camp David peace accords with Israel in 1978, Fahd and Sultan proposed continued close relations with the US, while Khaled and Abdullah, who was then a National Guard commander, advocated distancing Saudi Arabia from the US. Sultan was deeply involved in the US's covert campaign to provide money and weapons to the mujahideen fighting the Soviets in Afghanistan.
People who knew Sultan praised his "strategic vision, the capacity to think big", in particular after the 1973/74 oil price rises. Nevertheless, he had his critics. One analyst said that he presided over "the most colossal amount of money, in proportion to the size of a country's economy, ever poured down the barrel of a gun".
Sultan created a massive military establishment in Saudi Arabia through arms purchases from the US, the UK and France. He built military cities, largely with US support. However, the massive British-supported defence programme was also crucial. The 1988 al-Yamamah contract was described as Britain's largest ever export deal, worth at least £43bn, though its full extent has not been fully documented. It was later the focus of corruption charges in the UK and the US. Although Sultan was not named in these probes, members of his family were. According to David Holden in The House of Saud (1981), in the early 1970s "the majority share of transactions is said to have gone to Sultan in a ratio of about 60:40".
In 1990 US forces were deployed in Saudi Arabia to defend it against the Iraqi forces that had overrun Kuwait. His son Prince Khaled served as the top Arab commander in Operation Desert Storm, in which US, Saudi and other Arab forces drove the Iraqis out of Kuwait. In 2005, when Abdullah succeeded Fahd to the throne, Sultan became crown prince, despite rumours that he was not getting on with Abdullah.
Sultan had a reputation for a fierce temper but his habit of working deep into the night won him the nickname of "bulbul" – nightingale. He was both a conservative and political moderate. "Sultan," wrote Holden, "whose vigour on the couch was a cause for even more concern and respect, had proved a stern, tough and headstrong character."
For some time, he was said to have suffered from ill-health, having been diagnosed with colon cancer in 2004 and latterly affected by Alzheimer's disease. "Crown Prince Sultan has been for all intents and purposes incapacitated by illness for at least [the] past year," the US embassy in Riyadh wrote in a cable in May 2009, according to Wikileaks. He spent about a year abroad, recuperating in the US and at a palace in Agadir, Morocco, before returning to Saudi Arabia in November 2010, a day before King Abdullah himself went to New York for medical treatment after a blood clot complicated a slipped spinal disk.
Sultan had some 32 children by 10 wives. His son Bandar was Saudi ambassador to the US from 1983 to 2005, during which time he formed close relations with both presidents Bush.