Sunni-Shiite conflict has become the dominant political and strategic fissure in much of the Middle East.
The assembling of a Sunni coalition to challenge the advance of an Iranian proxy in Yemen, and the subsequent announcement in Sharm al-Sheikh of the formation of a 40,000 strong Arab rapid reaction force are the latest moves in a war which has already been under way in the Middle East for some time.
This is a war between Sunni and Shia forces over the ruins of the regional order. It is a war which is unlikely to end in the wholesale victory of one or another of the sides. 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 an attempt by the Sunnis to narrow the gaps in unity and effectiveness between themselves and their Shia opponents.
The Shia side is a united bloc, gathered around the structures of 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 its head. In their propaganda, they characterize themselves as an alliance of authentic Muslim forces, arranged against the west and its hirelings.
The Shia side is a united bloc, gathered around the structures of the Islamic Republic of Iran.
In reality, they are a gathering of almost exclusively Shia 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 assist in the Shia advantage in this regard.
In the Revolutionary Guards Corps and its Qods Force, the Iranians possess an instrument perfectly designed for the current moment in the region. This force is a gathering 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 their separate 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 and the Qods Force.
The Sunni side in this war has been far more disparate, confused and cumbersome.
Its existence and its skills are behind the domination of Lebanon by Hizballah, the survival of the Assad regime in Syria, the current Shia militia mobilization against the Islamic State in Iraq and the Ansar Allah (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 which gathers and directs all other forces under its wing.
For the last forty years, the most powerful Sunni Arab states formed the key components of the regional alliance headed by the United States. If Iran was the 'guiding' hand behind the Shia 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 seemed most inclined to grab them.
The status quo Sunni powers are now determined to organize themselves independently of the US.
What the events of the last week confirm, however, is that the 'status quo' Sunni powers, the once-allies of the United States, are now determined to organize themselves independently, given the absence of a US 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 United Arab Emirates are on board. The drive to halt the advance of the Iran-supported Houthis movement in Yemen is the first test of this new and unfamiliar gathering.
Success remains uncertain. Egyptian ships have been dispatched to the area. Air strikes have begun. But the wars of the present time in the Middle East are not primarily high-tech affairs. Air power certainly plays an important role. But in the end, these are grinding militia contests, fought out on the ground.
In such a war, the Shia 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 al-Qaeda affiliate (and Qatar client) Jabhat al Nusra of Idleb city in north west Syria.
Idleb 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 the Islamic State.
The Sunni mobilization is a sectarian gathering par excellence.
What this means is that the pushback against the Iranians as led by the Sunni Arabs is likely to involve Sunni jihadis, and Muslim Brothers (Hamas last week also declared its support for the Saudi initiative).
Nor has the Saudi initiative ended divisions among the Sunnis. The split between pro and anti Muslim Brotherhood elements has been only papered over. Earlier this month, Qatar and Turkey, the main MB-supporting Sunni states, signed a separate military accord.
This mobilization contains nothing in it of regional reform. It is a sectarian gathering par excellence.
But for all the cautions 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 high, perhaps historical significance. They represent the Sunni picking up of 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 in Yemen, its full dimensions have become plainly visible. A new chapter is beginning in the region.
Jonathan Spyer is Director of the Rubin Center for Research in International Affairs and a fellow at the Middle East Forum. He is the author of The Transforming Fire: The Rise of the Israel-Islamist Conflict (Continuum, 2011).