The "mother of all battles" has been an often used phrase in describing major military showdowns in the Middle East.

Perhaps the most famous, of course, was when President Saddam Hussein reiterated it as a warning shortly before the First Gulf War in 1991.

Since then it has been the hyperbola of choice by a number of leaders about to embark upon large-scale military campaigns many of which do not measure up to the gravitas of such a remark.

That cannot be said of the Middle East battle that every one knows is coming and could potentially mark a crucial turning point in the fight against the Islamic State (IS) group.

That this battle will be very bloody and the impact of its fallout profound is in no doubt.

I'm talking about the impending offensive to retake Iraq's second largest city Mosul. In what would be a protracted military campaign the number of casualties could be immense and aid agencies are already stockpiling in preparation for what could be as many as one million people fleeing the fighting in this region of northern Iraq.

It was last month that an anonymous senior source with US Central Command leaked details about what has been described as the "most widely anticipated military offensive in the Arab world".

The timescale remains open to speculation but the launch of any offensive could be as soon as April or May and will likely involve an Iraq security force consisting of 20,000-25,000 troops. To put this in some perspective and context this is more than twice the number of US soldiers deployed in the 2004 battle for Fallujah, which accordimg to the US military invloved "some of the heaviest urban combat US Marines have been involved in since the Battle of Hue City in Vietnam in 1968". Mosul will be much bigger, so just how are events likely to unfold?

To begin with, Iraqi forces are expected to advance from the south to retake the city while Kurdish peshmerga forces contain the city from the west, north and east. It is for this reason that Iraqi troops yesterday were battling for control of the city of Tikrit the capture of which would pave the way for a full push on Mosul.

One of the biggest problems facing these advancing forces is they will also be attacking a city where most of the civilian population remains in place. Unlike the IS-held cities of Fallujah and Tikrit, the IS has actively worked to ensure most Mosul residents remain trapped within the city. There the jihadis have enforced a "guarantee system" requiring those leaving the city to designate three hostages who will be punished if they fail to return.

This of course presents a particular set of challenges for the attacking force.

As veteran Middle East reporter Patrick Cockburn pointed out recently is is extremely unlikely IS would give up Mosul without a fight to the death.

Not only was it the city's unexpected capture by the jihadis last year that enabled it to proclaim the caliphate but its loss would drive home a devastating tactical and propaganda blow to its prestige and "sense that its victories are divinely inspired".

One only need to look at the recent battle for the town of Kobani where IS were able to hold out for three months against Syrian Kurdish fighters despite 700 US air strikes. Then there is the thorny question of just who would "liberate" Mosul.

Pulling together a unified force of disparate fighters numbering in the tens of thousands within a month is ambitious, to say the least, particularly when many of these groups collapsed last year and hold conflicting interests or historical enmity toward each other.

Washington has been quite vocal about taking the time needed to accomplish this goal before launching any offensive against Mosul, and US Secretary of State John Kerry was at pains to make that point earlier this week.

"We will do it when the moment is right and we know we can move forward with the confidence that we want," insisted Mr Kerry.

As seen from Baghdad, however, any offensive on Mosul is viewed differently. The Iraqi government is facing immense pressure from its citizens to show that the army is winning the fight against IS and is pushing for the fight to begin as soon as possible.

The composition of the fighting force also poses an issue for planners. While the Kurdish peshmerga, Iraqi army, Shiite militias and some Sunni tribal elements have been able to work together without too much friction thus far, the overuse of such forces in proximity to each other could result in infighting and collapse. And should they succeed in taking the city after a long drawn out campaign there are pressing questions over their ability to hold and consolidate in unity.

Would the Kurds want the Iraqi Army back on their doorstep? Would local Sunnis want a new flood of strangers from Shiite southern Iraq policing their city? What happens if Shiite militiamen and volunteers enter the fight or if any of the liberating forces begin to punish suspected IS collaborators within the population? These are just some of the relevant questions posed by analyst Michael Knights in a recent assessment in Foreign Policy magazine.

This too before the of issue of the potentially vast humanitarian fallout.

"We would expect hundreds of thousands of people from Mosul to leave, if not more," says Marwa Awad of the UN World Food Programme. She added that the numbers fleeing an impending battle for the city in the course of the next few months could total a million.

The "mother of all battles" may be upon such people all too soon. Few doubt it will be messy indeed.