The latest crisis in Iraq has been called many things.

One veteran Middle East reporter said it was "the most significant act by a jihadist group since al Qaeda attacked the US on September 11, 2001".

Another long-time watcher of the region put it much more simply, announcing that "Iraq is breaking up". Then there was the US-based terrorist-tracking think-tank that saw what happened in the Iraqi city of Mosul last week as "the world's most violent and valuable bank heist".

The simple truth of the matter is that all three assessments are accurate in their own right. More on those points in a moment, but what is also true is that events are moving very rapidly indeed.

Yesterday, Iraqi prime minister Nouri al Maliki sought to reassure army officers that help was on the way, as hundreds of young Iraqi men streamed into volunteer centres across the capital, Baghdad.

Meanwhile, Iranian president Hassan Rouhani also pledged support, saying Tehran stands ready to help Iraq's government in its fight against the Sunni Muslim insurgents of The Islamic State of Iraq and Syria (Isis), who have overrun Iraqi cities to within striking distance of Baghdad.

"Within the coming hours, all the volunteers will arrive to support the security forces in their war against the gangs of Isis. This is the beginning of the end of them," insisted Maliki, speaking in a broadcast on Iraqi television yesterday.

If only things were that simple, which takes me back to those original assessments by Middle East watchers. There is now no doubt about the significance of what Isis has been able to achieve since it launched its assault on Iraq's second largest city, Mosul, and Tikrit, birthplace of Saddam Hussein.

Writing in the influential Foreign Policy magazine in the wake of the Isis offensive on Mosul, analyst Michael Knights put its significance in perspective.

"In a matter of days between June 6 and 9, the city of 1.8 million people was overrun by Isis, the al Qaeda affiliate that broke away in April 2013 to fight its own war and which has come perilously close to achieving its dream of a caliphate that reaches from the Mediterranean coast of Lebanon to Iran's Zagros Mountains."

All this has happened barely two-and-half years after a US withdrawal that supposedly brought an end to the last Iraq War.

That Isis was sufficiently ­audacious, determined and equipped to launch its latest assault was flagged up as early as last December, when the rebels overran Fallujah, exploiting tactical miscalculations by the government to capture the highly prized city lost to US-Iraqi troops in a bloodbath some years ago.

Not surprisingly, this has led many to question the effectiveness of US intelligence operations in Iraq. The speed and ease with which well-armed and highly trained Isis fighters took Mosul and Tikrit have led to critics asking how the jihadists have once again so easily been able to "blindside" America's $50 billion intelligence community?

Observers point to the fact that since US forces left Iraq in December 2011, these intelligence agencies have become increasingly reliant on satellite imagery and communications intercepts rather than ground-level intelligence-gathering involving a network of spies.

Whatever the explanation, it has left the US looking at an Iraq that may indeed be in the process of breaking up.

We are not yet necessarily facing such a scenario, of course. But if Isis is able to consolidate its hold on Mosul and other cities seized, while the Kurds hold their own bulwark in the north and Shias around Baghdad and in southern Iraq remain in control there, then the country would effectively exist in a state of three-way partition.

Isis certainly appears to have the resources to hold its own for now, helped in part by the massive quantities of military hardware seized in Mosul and what a US think-tank, the Terrorism Research and Analysis Consortium (TRAC), called "the world's most violent and valuable bank heist."

With Iraqi troops abandoning their posts in Mosul, Isis was able to secure helicopters, armoured vehicles and an untold amount of arms and ammunition. But Isis also pillaged banks in the city, making off with a potential $3.5bn, says TRAC, which would make the Islamist rebels one of the richest terrorist groups ever seen.

With Iraqi soldiers and ­policemen melting away in the face of Isis's lightning advance, and thousands of civilians fleeing to the self-ruled Kurdish region in northern Iraq ruled from Erbil, the situation remains dangerously fluid.

Yesterday, Isis fighters seized the small town of Adeim in Diyala province, 60 miles north of Baghdad, after Iraqi security forces again pulled out.

This fast-moving rebellion has emerged as the biggest threat to Iraq's stability since even before the Americans left and once again rendered Iran's western border with Iraq vulnerable.

For the moment there is the sense that all sides - Isis, Baghdad, Erbil, Washington and Tehran - are taking stock of their respective positions. Unpredictable and potentially catastrophic days lie ahead all round.