David Usborne: Washington knows it can do very little to rein in Saudi repression

The Saudi leaders have not yet got over how the US cut loose Egypt's Hosni Mubarak, and US-Saudi relations are as testy now as at any time since the 9/11 attacks

If Washington is uneasy about a Saudi-led foreign force of 1,000 soldiers crossing the causeway from Saudi Arabia into Bahrain, it is nonetheless keeping its criticism muted for two reasons. Firstly, it would welcome an end to the turmoil in the island nation, home to the Fifth Fleet, and secondly, it knows that wagging its finger at Riyadh right now is unlikely to have much effect.

No Arab nation is more important to the US than Saudi Arabia. It remains a bulwark against Iranian expansionism and a pivotal partner fighting global terrorism. Thus the statement from the White House stressed that the deployment of troops is "not an invasion" and the mild admonition only that they show "respect and restraint" on Bahraini soil.

In any event, anything stronger would not fly because not since the post-9/11 days – when it emerged that most of the hijackers had come from the Kingdom – have Saudi-US relations been as testy as they are today.

It probably hasn't helped that in recent weeks the US has tightened the tone of its criticisms of Saudi Arabia by saying that to stave off trouble of its own, Saudi needs to pick up the pace of the social and political reforms it has allegedly been pursuing for the past several years, but which have been so subtle as to be almost invisible.

Mostly, though, the Saudi leaders have yet to get over how America handled last month's crisis in Egypt. As the protests intensified in Cairo, the Saudis implored President Barack Obama to stand by their ally, Hosni Mubarak. Egypt, after all, is itself hardly peripheral to US strategy in the Middle East. Mr Obama nonetheless cut Mubarak loose anyway. Raising the temperature now is hardly what Washington wants. It is Saudi Arabia which in recent days has increased oil production to offset the loss of supplies from Libya, helping to avert a global energy shock. And in truth, the overlap of interests between the two countries remains significant as the turmoil in the Middle East persists.

Both want calm to return to Bahrain, neither wants an uprising to take shape in the Kingdom itself, and both want to limit Iran's influence in the region.

Mr Obama leads a country that considers itself a beacon of democracy in the world. Saudi Arabia, by contrast, remains a jarringly conservative society led by a ruling family that has no interest in power-sharing and has responded to the first glimmerings of unrest within its borders by buying off the population with fresh welfare goodies and with the iron fists of police action and press censorship.

If Saudi Arabia deemed it necessary to send troops as part of a Gulf Co-operation Council force into Bahrain to protect the monarchy there and to fend off Iran from extending its influence on the peninsula, it was never going to ask permission from the US first. Never mind that its longer-term interests are keeping the US on side as its main supplier of arms and medical care to the ailing King Abdullah.

And that leaves Washington rehearsing all the usual bromides about restraint and respect. But this horse is already out of the stable – or across the causeway – and the US could do nothing about it even if it wanted to.