Crown Prince Sultan bin Abdul-Aziz Al Saud: Heir to the throne of Saudi Arabia

Crown Prince Sultan bin Abdul-Aziz Al Saud was heir to the throne of Saudi Arabia and for decades one of the most powerful personages in the oil-rich and centrally important Middle Eastern state. As his age, officially given as 81, indicates, he was a senior member of the Saudi gerontocracy.

The ill-health of its 87-year-old ruler, King Abdullah, his half-brother, means the succession is likely to pass to another of Abdullah's half-brothers. This is interior minister Prince Nayef bin Abdul-Aziz, who is aged 78. As this suggests, power in the enormous, thousands-strong family which controls Saudi Arabia traditionally passes from brother to brother rather than from father to son. While this has so far made for a certain continuity, the question arises of when power will shift to younger members of the family.

The Saudi authorities are anxious to project that the Arab Spring will not cause the sort of turbulence in their country which has broken out in other parts of the Middle East, and to show that the eventual succession will take place in an orderly manner. But Sultan's death, after some years of battling cancer, comes at a delicate moment for a ruling family which has to balance its close financial links to the US with a recognition of anti-American sentiment in its region.

Born in Riyadh, Sultan was a son of the founder of the kingdom, King Abdul-Aziz Al Saud, and the most powerful of his 32 wives, Hussa Bint Ahmad Al Sudayri. Sultan was one of his mother's seven sons, known as the Sudayri seven. In all, he had around 45 brothers, who were educated at the royal court in religion, culture and diplomacy. He also learnt English.

His rise through the ranks saw him becoming, in 1947, governor of Riyadh. Joining the cabinet, he headed the ministries of agriculture and communications before in 1963 becoming minister for defence, a post he held for decades. In that post he presided over enormous purchases of military hardware, especially from the US and Britain. In the years that followed, he was the key figure in a massive build-up of the Saudi military establishment, spending huge amounts of the country's oil revenues on aircraft and other equipment. The US and British aerospace industries profited greatly from contracts worth many billions.

As a result of this, Sultan and other members of his family developed close relations with the US. In particular, his son, Prince Bandar Bin Sultan, became an important link-man, serving as ambassador to Washington for many years, where he maintained personal contacts with a series of US Presidents. Sultan himself, once described in his earlier days as "a volatile and emotional young man", came in later years to be regarded as a safe pair of hands, a tough negotiator and blunt speaker.

His fondness for the company of women was noted at an early age, and he went on to have many wives, who provided him with 32 children. His family nickname of "bulbul", meaning nightingale, came from his habit of conducting business late at night.

Strains arose in US-Saudi relations over the US attitude towards the Palestinian question, and because of the fact that a majority of the 9/11 hijackers were Saudi citizens. Sultan's response was to criticise the US over Palestine, but simultaneously to denounce Bin Laden as "a deviant" who wished to drive a wedge between America and Saudi Arabia. In a rare New York Times interview in 2002 he attempted to strike a balance. "In the current environment, we find it very difficult to defend America, and so we keep our silence," he declared. "Because, to be very frank with you, how can we defend America?"

But he insisted that the Saudi relationship with the US remained "very strong". He said: "I have been accused of being critical of the United States. My response is – ask the Presidents who have dealt with me, President Carter, President Reagan, President Bush, President Clinton and President Bush Jr., with whom I expect to develop a strong relationship."

In other words, he was signalling that any differences would not be allowed to disrupt an association which was so mutually beneficial in terms of oil and arms procurement. The continuation of the relationship was illustrated when, at his death, US Secretary of State, Hillary Clinton, spoke of Sultan as "a strong leader and a good friend to the United States over many years as well as a tireless champion for his country."

His importance in the ruling family was affirmed in 2005 when King Abdullah came to the throne and designated Sultan as crown prince and heir. This was seen as confirmation both of his abilities and his skill at manoeuvring within the family, where the Sudayri seven were thought to cooperate effectively together.

Sultan's health issues became serious in 2004, when he was diagnosed with colon cancer. He spent lengthy periods in New York under treatment for the condition, recuperating in the US and Morocco, with his duties being delegated to other family members. He was also reported to have developed Alzheimer's disease. The full extent of his problems emerged in 2009 when an American diplomatic cable released by WikiLeaks contained a confidential assessment that he was "to all intents and purposes incapacitated".

Sultan bin Abdul-Aziz Al Saud, politician: born Riyadh, Saudi Arabia 1926; married ten times (32 children); died New York 20 October 2011.