By Jonathan Marcus
BBC Defence Correspondent
The upsurge of violence in Saudi Arabia with its implicit threat to oil prices is the last thing the US Administration wants right now, as it continues to struggle against the insurgency in Iraq.
Best of friends? The US and Saudi Arabia have a complex relationship
The political violence in Saudi Arabia is directed as much at the Saudi state as it is against foreign expatriate workers in the kingdom.
But in the Bush Administration's broad geo-strategic view, building democracy in Iraq and promoting reform in Saudi Arabia are linked.
"It's about the oil, stupid!" That was the blunt comment of many opponents of the US-led invasion of Iraq.
The critics did not buy the reports about weapons of mass destruction.
They were even less impressed by the claims to be bringing democracy to the Iraqi people.
This, they believed was an old-fashioned struggle about that most combustible of natural resources - oil.
At one level the critics were making an obvious point.
It is not the territory of the Middle East that makes it such a strategic asset but what is under it that counts.
Oil is certainly one of the reasons why Iraq matters to Washington.
But in the minds of the neo-conservatives who helped launch this war, Iraq was firmly linked to the future of America's long-standing ally in the region, Saudi Arabia.
Relations between Washington and the Saudi monarchy have always been complex, even ambiguous.
But the attacks in New York and Washington of 11 September 2001 revealed this relationship's full ambivalence.
Many American policy-makers believed that Saudi Arabia had done far too little to curb the rise of al-Qaeda.
The US invasion of Iraq enabled Washington to pull its troops and air force squadrons out of Saudi Arabia.
This removed a major irritant fuelling opposition to the Saudi ruling elite.
At the same time the US hoped that the Saudi authorities would seize the opportunity and clamp down on al-Qaeda once and for all.
But many US experts wonder if even now the Saudi elite really has the will to crush al-Qaeda and its sympathisers.
It is just one more problem in a region where Washington already has its hands full.