There have been clear signs in recent weeks that Saudi Arabia, America’s principal Arab ally in the Middle East, has become deeply concerned at the trend of events in Iraq. The Baker report came as a shock in Riyadh, where it was seen as a pointer to a change in US policy — a change that has unwelcome implications for the Saudis. They are already bracing themselves for a fresh confrontation with Iran — over Iraq, but also over Tehran’s determination to develop a nuclear weapon. And Saudi Arabia, like much of the Arab world, is beginning to suffer the effects of the confrontation between Sunnis and Shias in Iraq, and is rightly fearful of the destabilisation that this violence could bring to the region.

The White House has denied reports that King Abdullah warned the Bush Administration two weeks ago that Saudi Arabia might provide financial aid to the Sunni minority in Iraq if American troops were withdrawn. But the denial does little to undermine the assumption that Riyadh will have to come to the aid of beleaguered fellow Sunnis in Iraq. A similar warning was made three months ago by an aide to the Saudi Ambassador in Washington. The Baker report can only have increased concern. The Saudis were piqued that it gave barely a mention to their role in any Iraqi settlement and concentrated instead on the need to involve Iran and Syria. They feel that their role as a steadfast military ally has been taken for granted in Washington and that their influence on Iraq’s Sunnis, far greater than that of Syria, is being overlooked.

Riyadh is also deeply worried that the Baker report will be used in Washington to prepare the way for a swift withdrawal of US troops — despite White House denials. Given the very high level of sectarian violence, a US pullout would be extremely destabilising and could signal the start of an all-out civil war that would inevitably suck in Iraq’s neighbours on different sides.

For the Saudis, the overriding danger is Iran. Decades of suspicion and hostility have not been overcome, despite recent declarations of good neighbourly intent. The Saudis were aghast at the election of the hardline President Ahmadinejad. His subsequent statements and confrontational policies have exacerbated their fears. And Iran’s growing interference in Iraq and open support for Shia extremism have fuelled suspicion that Tehran is determined to expand its influence, backed up with the threat of nuclear weapons. Any break-up of Iraq would lead, the Saudis believe, to the establishment of a Shia client state that Iran would use as a base for political and religious infiltration throughout the Gulf. So, in a clear signal to Iran, the Saudis have just announced a joint programme with members of the Gulf Co-operation Council to develop nuclear energy — the plan is “peaceful” but the ultimate outcome could be far more troubling.

Given Saudi nervousness, the sudden resignation of Prince Turki al-Faisal, their urbane ambassador in Washington, could not have come at a more awkward moment. There is no suggestion that he had fallen foul of the Bush Administration, but there are hints at disagreements over his role. The US needs to be conscious of its ally’s concerns. As the violence in Iraq continues, Washington needs the experience and counsel of friends in the region. Any serious split with Saudi Arabia could have serious consequences.