For many years, the world has looked at the Middle East in fear that any conflict there could spill over into a region-wide war that pits the two great sects of Islam against each other.
On the weekend the Sunni Arab states announced that they were forming a joint army, a standing force of some 40,000 elite troops.
Their most urgent target: the agents of the superpower of the Shia world, Iran.
Giving concrete force to their intention, the leading Sunni powers of Saudi Arabia and Egypt last week launched a joint assault on Iran's agents in Yemen, the Houthi fighters who've taken control of the capital, Sanaa.
Iran is rising, and the Sunni Arab states have had enough. As the Saudi Foreign Affairs Minister, Saud al-Faisal, has said: "We see Iran involved in Syria and Lebanon and Yemen and Iraq and God knows where."
While the Saudis and Egyptians are their leading powers, the wider bloc of Sunni-majority nations includes Jordan, United Arab Emirates, Qatar and Iraq.
A group of the Sunni Arab nations are taking an active part in the air war against the Houthis in Yemen.
Yemen is the world's latest failed state. But Saudi Arabia shares a 1800 kilometre border with its southern neighbour and has no intention of allowing it to become an Iranian playground.
The two sides of this sectarian divide, the Sunni and the Shia, are not attacking each other's territory directly, but they are fighting an intensifying proxy war across several fronts.
A question that springs immediately to mind for Australia as it seeks a bearing – which side is America on? The answer: both.
On one hand, the US is the chief ally and longtime protector of Saudi Arabia and its Sunni Arab allies.
On the other hand, the US is allied on the same side as Iran as they fight to regain control of Iraq from the so-called Islamic State forces.
Further, the US is leading the international negotiations with Iran seeking agreement over its nuclear program, with the deadline for a deal in the next 24 hours.
In fact, that's one of the key reasons that the Sunni Arabs have decided they need to create their joint army and act decisively against Iran.
The Saudis and other Sunni Arab states are frantic that the US is entering into a close relationship with the Ayatollahs:
"In recent weeks dozens of articles in the Arab press, and particularly in the Saudi press, have harshly criticised the Obama administration's policy in the region – especially its Iran policy, which they term 'destructive', 'idiotic', 'dangerous' and 'narrow-minded'," summarises the Middle East Media Research Institute.
Their fear? That the US will reach a nuclear deal with Iran, and this will then lead them on to reach a deal over the civil war in Syria. Iran is the chief ally of Syria's Bashar al-Assad.
In this, the Saudis and other Sunnis share the apprehension of the Israelis. As its prime minister, Benjamin Netanyahu, said to the US Congress: "In the Middle East, Iran now dominates four Arab capitals – Baghdad, Damascus, Beirut and Sanaa. And if Iran's aggression is left unchecked, more will surely follow.
"So, at a time when many hope that Iran will join the community of nations, Iran is busy gobbling up the nations."
Iran, interestingly, applauded the Israeli leader for recognising the sweep of its growing power. "Netanyahu acknowledged with certainty Iran's might and influence; he said that Iran has taken over four countries in the region," said an adviser to Iranian President Hassan Rouhani and former intelligence minister, Ali Younesi, according to the state-owned news service.
All sides agree on Iranian imperial ambition. And all US allies in the region, Arab and Jew alike, converge in deep doubt over American willpower and wisdom.
The collapse of Yemen is cited as yet more evidence of failing US policy. A year ago, Barack Obama lauded his Yemen strategy as a model for US counterterrorism. The US waged a drone war on terrorist targets with the help of a sympathetic government.
But now the US-backed government in Sanaa has fallen, the president has fled to Riyadh, the Iranian-supplied Houthis have taken over, and US special forces detonated tonnes of sensitive equipment on March 20 and fled the Houthi onrush.
The Sunni Arab states have a second reason for forming their combined army – they are threatened by so-called Islamic State.
Although IS is notionally Sunni, the governments it most hates are those of the Sunni Arab countries because they are the most guilty of betrayal of Islam, they claim.
The natural temptation faced with such an intensifying series of complex threats is withdrawal. Yet that's the last thing that the US and its allies should do, according to Martin Indyk, the Australian-born US Middle East expert:
"Not taking a stand in Syria was the original mistake that helped to open the gates of hell." Intelligent involvement, not inaction, is the right response.
Yet the acid test for Australia, as the Lowy Institute's Anthony Bubalo puts it, is this: "The whole regional order has broken down. We have very little influence. The question for us: 'How smart is the guy whose action we're joining? How good is his policy?'."
The evidence on the ground makes it increasingly difficult for US allies to put their hands on their hearts and express full confidence.
Peter Hartcher is the international editor Sydney Morning Herald