Salman makes his play in a Saudi game of thrones

Four months into his reign, King Salman bin Abdulaziz al-Saud of Saudi Arabia has shown the world what kind of ruler he intends to be. The 79-year-old has reshuffled the upper reaches of his kingdom in a decisive and surprising shift of power. The changes - which include casting out his half-brother as crown prince and nominating his two successors - should bring more stable and vigorous rule than the gerontocratic monarchy has managed in decades. The question is whether they also herald a more confrontational foreign policy from Saudi Arabia at a time of turmoil across the Middle East.

The Saudi elite has always been secretive and plagued by rival family factions. By appointing his nephew Mohammed bin Nayef as crown prince and his favourite son Mohammed bin Salman as second in line to the throne, the king has firmly lined up the future succession. At 55, the new crown prince is on course to be the first grandson of Ibn Saud, the kingdom's founder, to rule the nation. This marks a historic shift away from the tradition whereby rulers were drawn from the direct offspring of Ibn Saud himself. Riyadh's western allies have long urged this change for the sake of stability.

Inevitably, this overhaul prompts as many questions as it does answers. Prince Mohammed bin Nayef is well known in the US and recognised as a pivotal figure. But the internal challenges facing Saudi Arabia are huge. Many in the west hope the ultraconservative kingdom will embrace greater religious freedom and create more jobs for its rapidly expanding population. Yet Prince Mohammed's willingness to reform is unclear.

It is in foreign policy that the appointments may be more revealing. Saudi Arabia has been fighting an increasingly aggressive proxy war with Iran in the Middle East, backing Sunni regimes and trying to counter Iranian influence in Syria, Iraq and Bahrain. Mohammed bin Salman, the new deputy crown prince, has led the prosecution of Saudi Arabia's bombing campaign against Iran-backed Houthi rebels in Yemen. His elevation may signal that this bold policy is about to be extended further afield.

If so, then the Saudi leadership should think again. In Yemen, Riyadh has given a brutal demonstration of its air power and marshalled a range of Arab nations to back its effort. But air strikes have not weakened the Houthis and a political settlement remains beyond reach. Nor has Saudi Arabia much cause for celebration in its fight against Iran across the region. The kingdom can claim some victories, notably the return of a military-backed government in Cairo. But its lavish funding of proxies has not yielded stability in Syria, Iraq or Libya.

The changing of the guard in Riyadh should prompt some hard thinking in Washington about its Middle Eastern policy. The Obama administration is rightly pressing for a deal with Iran that would reduce sanctions on Tehran in return for a curbing of its nuclear programme. But the White House should accompany this with a renewed effort to forge common ground between Iran and Saudi Arabia, ensuring that any nuclear pact does not heighten tensions across the Gulf. Next month's summit between Mr Obama and Gulf leaders is an important moment for frank discussions.

The Obama administration has long wanted Saudi Arabia to play a bigger role in the Middle East, and has encouraged Arab rulers to take greater responsibility in managing local problems. The kingdom's new leaders need to do this in a way that consolidates stability rather than setting fresh fires in a region that is already ablaze.

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