The Saudi-organized coalition for Yemen and the announcement of a regional Arab force show that the Sunni states have finally picked up the gauntlet thrown down by the Iranians.

The assembling of a Sunni alliance to challenge the advancement of an Iranian proxy in Yemen, and the subsequent announcement in Sharm e-Sheikh of the formation of a 40,000 strong Arab rapid reaction force, are the latest moves in a war that has already been under way in the Middle East for some time.

This is a war between Sunni and Shi’ite forces over the ruins of the regional order. It is a war that is unlikely to end in the wholesale victory of one side. Rather, it will end when the two forces exhaust themselves. What the region will look like when this storm passes is anyone’s guess.

The two sides in this war differ in significant ways.

The Saudi and Arab League announcements constitute the Sunnis’ attempt to narrow the gaps in unity and effectiveness between themselves and their Shi’ite opponents.

The Shi’ite side is a united bloc, centered around the Islamic Republic of Iran. The Iranians are an overtly anti-Western and anti-status quo force, seeking a new Middle East order with themselves at the head. In their propaganda, they characterize themselves as an alliance of authentic Muslim forces, organized against the West and its hirelings.

In reality, they are a gathering of almost exclusively Shi’ite groupings, but a cohesive and united one.

It is possible that the traditions of clandestinity and cross-border communication of a long subaltern regional minority gives the Shi’ites an advantage in this regard.

In the Revolutionary Guards Corps and its Quds Force, the Iranians possess an instrument perfectly designed for the current moment in the region. An army of professional revolutionaries whose specific trade is the mobilizing and direction of proxy political- military organizations.

The context of the current war is one in which states have collapsed and separated into sectarian components. In Yemen, Iraq, Syria and in a less kinetic way Lebanon, would-be “successors” to the state organized on a sectarian or ethnic basis are fighting one another.

In such a context, the existence of a state agency whose specific field of expertise is the creation and maintenance of sectarian political-military organizations is an enormous advantage. The Sunnis have no equivalent of the IRGC.

Its existence and its skills are behind Hezbollah’s domination of Lebanon, the Assad regime’s survival in Syria, the current Shi’ite militia mobilization against Islamic State in Iraq and the Houthi offensive in Yemen. The Sunni side in this war has been, since its inception, a far more disparate, confused and cumbersome affair.

There are a number of reasons for this. There is no Sunni equivalent of Iran, no single powerful state that can gather and direct all forces under its wing.

For the last 40 years, the most powerful Sunni Arab states formed the key components of the regional alliance headed by the US. If Iran was the “guiding” hand behind the Shi’ite challenge to the regional status quo, then the organizing force behind the pro-status quo Sunni states was the US.

But in the last half decade of emergent sectarian war in the region, the United States has been absent, entirely unaware of the dynamic of events. So the Sunnis have been adrift.

The US has sought to appease both the Iranians and the radical, anti-Western element among the Sunnis – the Muslim Brotherhood. All this apparently as part of an effort to withdraw from the region and leave the keys with whoever seems most inclined to grab them.

What the events of the last week confirm, however, is that the “status quo” Sunni powers, the once-allies of the US, are now determined to organize themselves independently given the absence of an American guiding hand.

The commitment of nine Sunni-majority countries to the Saudi-organized alliance is the fruit of an ambitious attempt by Riyadh to create a new, regionally led counter-bloc to the Iranians.

Morocco, Egypt, Jordan, Sudan, Pakistan, Qatar, Kuwait, Bahrain and the United Arab Emirates are on board. The drive to halt the advance of the Iran-supported Houthis is the first test of this new and unfamiliar coalition.

Success remains uncertain. Egyptian ships have been dispatched to the area. Air strikes have begun.

But the wars of the Middle East today are not high-technology affairs. Air power certainly plays an important role, but in the end, these are grinding contests, fought out on the ground.

In such a war, the Shi’ite Islamist and tribal guerrillas of the Houthis and their IRGC guides are likely to enjoy a certain advantage.

The difficult terrain of Yemen is likely to exacerbate this. This raises a further difficulty for the Sunnis.

So far, the experience of Iraq and Syria indicates that the only Sunni forces that have gone toe-to-toe with the Iran-backed element and held their ground are Islamists.

Note the recent conquest by a force led by the al-Qaida affiliate (and Qatar client) al-Nusra Front of Idlib in northwestern Syria.

Idlib is the second provincial capital to fall to the anti-Assad forces in four years of civil war. The first was Raqqa, further east. It’s now controlled by Islamic State.

What this means is that the pushback against the Iranians, as led by the Sunni Arabs, is likely to involve Sunni jihadis and the Muslim Brotherhood (Hamas last week also declared its support for the Saudi initiative).

But the Saudi initiative hasn’t ended divisions among the Sunnis. The split between pro- and anti-Muslim Brotherhood forces has been only papered over. Last month, Qatar and Turkey, the main Brotherhood-supporting Sunni states, signed a separate military accord.

This mobilization contains nothing in it of regional reform. It is a sectarian alliance par excellence.

But for all the warnings and caveats, the emergence of the Saudi-organized coalition for Yemen and the announcement of the new Arab force to deploy in the region are developments of great, perhaps historical significance.

They represent the Sunnis picking up the gauntlet thrown down a while back by the Iranians. This war was a long time coming.

It emerged in stages. It has been here for a while. This week, with the announcement of the Saudi-led alliance, its magnitude has become plainly visible. A new chapter is beginning in the region..