Why angry allies may see this as signs of a superpower in retreat

Washington feels triumphant, but has a superpower ever struck a deal like this, in defiance of its most closely affected allies? The United States is still, despite recent debacles, the global hegemon. Ignoring the pleas of allies such as Israel and Saudi Arabia is just one of those things a superpower can do if a president wants to put a diplomatic achievement in the White House column marked "legacy".

But for Sunni Arab leaders, deeply hostile to Iran, the deal is a sign that America's power is waning, that it has suddenly funked, decided it needs a multipolar world: one in which Washington negotiates with rather than rides roughshod over regional rivals such as Iran, and global rivals such as Russia and China.

Those leaders are now fast reassessing their own relations with Moscow and Beijing. If you believe the noises coming out of Riyadh and Jerusalem, after the deal comes the deluge. Even if Israel holds off from bombing Iran's nuclear facilities - and it was ruling nothing out yesterday (Sunday) - other disasters are lining up. Most immediately, a newly respectable Iran will be emboldened in its support for President Bashar al-Assad of Syria. Already, thousands of Iranian, Lebanese and Iraqi Shia militiamen are there, helping Mr Assad in his war against the Sunni majority.

Either Iran will win Mr Assad a diplomatic victory in peace talks, allowing him to remain in power, or Saudi Arabia and other Gulf donors will increase their backing for the rebels, allowing al-Qaeda to increase its already substantial presence. In either case, many more Syrians will die. More quietly, convinced that Iran's promises are hollow and that it can no longer trust the United States, could Saudi Arabia itself go nuclear?

It too has never ruled this out. Pakistan's nuclear deterrent has been described as "Saudi's proxy bomb" because of the two nations' close co-operation; that might become reality. Meanwhile, it remains the world's number one oil producer and has recently become the main guarantor of Egypt's new military-backed regime. It seems quite happy to see Cairo currying favour and arms deals with Russia.

The Sunni world, it seems, may be on the verge of falling out of the American orbit. Of course, Washington's wonks are aware of the unhappiness the deal has generated. They are, almost certainly rightly, sceptical of the wilder Israeli and Saudi threats: in reality, these countries are angry precisely because they have nowhere else to turn. There are also other potential outcomes.

If Saudi Arabia and Israel shared notes over Iran, they might also notice a common long-term interest in resolving the Palestinian issue. Saudi is a firm backer of the Palestinian Authority in the West Bank, which the West prefers to Hamas in Gaza.

If Iran were more secure in its relations with America, it might feel its relationship with a sarin-deploying dictator overseeing the break-up of Syria to be less in its long-term interests. If the Gulf states did indeed start taking responsibility for more of their own security, fewer American troops might end up in harm's way. Such win-win outcomes are, however, something of a rarity in the Middle East.

The simple truth may be that the deal will have fewer immediate consequences than imagined. On all other issues, the West and the Iranians will continue to work against each other, and the violent schisms in the region will continue to worsen.

And in fact, underlying everything is an even simpler truth: Western voters are more worried about avoiding future wars than ending current ones. Supporting Saudi Arabia does't win anyone votes in swing states. The Geneva deal fits the isolationist mood currently dominant in both the US and Europe. Isolationism has backfired in the past, and may yet do so again, but that is for another day.