Poor old Barack. Off he goes to Riyadh to talk to his so-called ally, Saudi Arabia. The Sunni Wahhabi kingdom long ago run out of patience with the US president, who befriended Shiite Iran and who failed to destroy the Alawite (read: Shiite) regime in Syria. So why is Obama even bothering coming to the Gulf? Does he have any friends left among the kings, emirs and princes of Saudi Arabia, Kuwait, Bahrain, Qatar, the Emirates and Oman?

Obama won’t be entering the Saudi lions’ den. The Saudis were never as brave as lions – which is why they let the decidedly unprincely Osama bin Laden lead the Arab legion in Afghanistan – but the little cubs now trying to run the country are very angry.

The ambitious, ruthless deputy crown prince and defence minister, Mohamed bin Salman, launched the kingdom’s crazed war against the Houthi rebels in Yemen last year, convinced (without evidence) that Iran was arming them. The young Saudi foreign minister Adel al-Jubair – brilliant former Washington ambassador, a man with a silken, dangerously eloquent tongue – has no hesitation in denouncing Western weakness.

And, according to the New York Times, the Saudis have even threatened to sell billions of dollars of their US assets if Congress passes a bill allowing the Saudi government to be held responsible in American courts for the crimes against humanity of 9/11.

And that, indeed, is the foundation of the US-Saudi mess right now. Of the 19 hijackers involved in 9/11, 15 were Saudis, a fact diplomatically ignored in the years immediately following the attacks. The Saudis bankrolled the Taliban for many years.

The Americans believe – rightly – that Isis itself today receives much support from within Saudi Arabia, though they haven’t gone quite so far as to say the government is behind this. Saudi Arabia, in other words, is regarded in Washington as a very dodgy nation to be an ally.

But Obama’s got to pretend to King Salman (the crown prince’s Dad) that the US still stands four-square behind the kingdom’s security and sovereignty – he can hardly say he’s going to support Saudi ‘democracy’ for obvious reasons – and it’s clear that the country’s massive oil reserves, its million barrels a day output, strategic location and control of Sunni Muslim finances, means that the West has got to go on paying obeisance to all the regional head-choppers.

Be sure that when King Salman dies (and may he live for many years), David Cameron will once more lower the Union flag in mourning as he did for his predecessor.

The real problem is that – after years of fantasy in which, against all the evidence, the Americans persuaded themselves that the Saudis were a ‘force for moderation’ in the Middle East – the Obama administration has decided that Shiite Iran and the huge influence it exerts over the Shiite governments of Iraq and Syria (and over the Shiite Hizballah in Lebanon) is a better bet than the Sunni Salafists of Arabia. Hence the nuclear deal with Tehran’s new leaders, the end of sanctions against Iran and the slowly-dawning realisation among Sunnis that Washington is going to tolerate the continuation of Bashar al-Assad’s rule in Damascus.

Iran may, as it was under the Shah, become the policeman of the Gulf. The Saudis will have to share power with them. The US wants no more “free riders” (as Obama snottily described the Saudis) supporting Isis.

The Obama line, which will be peddled heavily this week, is that diplomacy rather than war must resolve the Sunni-Shia conflict; that America is not going to embark on any more military adventures in the Middle East (nor, one suspects, give much more support to Crown Prince Mohamed’s adventure in Yemen).

It would be good to know what the censored 28 pages of the official US 9/11 report said about the Saudis. Maybe Obama will mention that in Riyadh? Any more talk of withdrawing billions of US assets might just persuade the Americans to open the book and let us take a peek into those secrets.