In the past week, the Peshmerga armed forces of Iraqi Kurdistan put a major dent in the northern defences of Mosul, Iraq's second largest city and capital of the self-styled Islamic State of Iraq and the Levant (ISIL). The Peshmerga launched a powerful offensive on both sides of the Tigris River to the north of Mosul, extending the area of Kurdish control around ISIL strongholds like Kisik (the former base of the 3rd Iraqi Army division), Wana and Badush.

The Kurds are now 32km northwest of Mosul city to the north and are much closer, often just 8 to 16km, from the eastern areas of Mosul city. Along the Syria-Iraq border the Kurds are gradually extending their control around Sinjar and restricting ISIL use of the border areas closest to Mosul.

The federal government's main forces are just over 160km to the south, firming up their control of Beiji, site of Iraq's largest refinery and a vital crossroads that links ISIL areas of strength in Anbar, Kirkuk, Tikrit, and Mosul.

With international air support and intelligence, the federal government's special forces are periodically probing the desert areas west of Mosul with a view to choking off the ISIL line of supply to Syria.

Battle for Mosul in 2015?

A new nine-brigade Iraqi army force is being slowly assembled by Iraq's Ministry of Defence with US backing, intended to train and equip 45,000 troops specifically for the task of urban assault in the face of heavy street-by-street resistance.

At the same time another war is being fought largely unseen - the war of the coalition's spies and sensors versus ISIL's sentinels keeping a close eye on the citizens of Mosul.

The US and other international allies can map every structure and track every signal emanating from Mosul.

Local informants talking to the Kurds, Iraqis and Americans are helping to build a picture of life inside Mosul and the location and habits of ISIL in the city.

All these preparations are being made in advance of the main event; a storming of Mosul city during 2015. But when will this attack take place and how long will the battle for Mosul last?

For the federal government in Iraq, time is of the essence.

Baghdad's leaders want to deliver tangible victories against ISIL in 2015, and that means liberating ISIL-held cities. Iraqi leaders may be tempted to view Mosul as the "head of the snake", the ISIL capital within Iraq and a far more significant and populous city than ISIL's first capital in Raqqa, Syria.

ISIL would not disappear with Mosul's recapture, but a powerful blow would be struck against its prestige and recruitment potential. ISIL can probably muster well under 10,000 militants in a city of nearly one million residents, meaning that the balance could turn against them rapidly if the populace feels that liberation is close at hand.

Call for an early probe

These factors have led some Iraqi government planners to call for an early probe of the Mosul defences, to test whether ISIL really can control the city in the face of an imminent government offensive.

An alternative, slower approach to the liberation of Mosul is based on a different appreciation of the situation on the ground in the city.

ISIL and its predecessors have proven effective at urban defence, in the past during the 2004 battles of Fallujah and more recently in Syria at Aleppo and in Iraq at Tikrit. ISIL is actively forcing the population to stay inside Mosul, complicating the risk of civilian casualties in any hasty attack on the city.

ISIL and its predecessors have proven effective at urban defence, in the past during the 2004 battles of Fallujah and more recently in Syria at Aleppo and in Iraq at Tikrit.

ISIL is actively forcing the population to stay inside Mosul, complicating the risk of civilian casualties in any hasty attack on the city.

Widespread use of crude homemade landmines gives ISIL the ability to slow down the attackers as they laboriously clear mined areas.

Mosul is a large city, 26sq km, not substantially smaller than Baghdad in terms of its surface area. This means Mosul is unlikely to be secured by the limited federal forces available today.

ISIL seems to have maintained effective control over the local population in Mosul until now, though that may change when government forces draw closer.

Finally, it will be tough to isolate Mosul from Syria entirely because desert areas to the west - Ain al-Jahsh, Tall Abta, Tall Afar, Baaj - remain under ISIL control and will require both ground forces and airpower to interdict.

The Kurds have held the closest positions to Mosul city ever since ISIL overran Mosul in June 2014. In fact, to the northeast of Mosul, Kurdish forces have never been more than 13km from the centre of the city throughout the past seven months.

Now Kurdish forces make up the jaws closing on Mosul from the north and the south, but these jaws are perhaps unlikely to close entirely. The Kurds are keeping ISIL under pressure and limiting their ability to reinforce Mosul but that does not mean the Kurds are willing to suffer heavy casualties in street-to-street fighting to retake the predominately Arab areas of Mosul city, which account for almost all of western Mosul and significant neighbourhoods east of the Tigris.

Who will 'liberate' Mosul?

The federal government probably has to be the primary force-provider for the attack on ISIL in Mosul. If Baghdad is willing to wait until the middle of the year, or even beyond, the US will probably help to develop more powerful assault forces needed to evict ISIL, and the new police forces to reestablish control in the city.

If Baghdad wants to move sooner than the late summer there are only two options; first, a daring but extremely risky "thunder run" into the city whereby small special forces and tank units try to spark an uprising against ISIL.

The only other near-term alternative is reliance on predominately Shia Hashd al-Shaabi (Popular Mobilisation) units.

But these forces are arguably too small and too distracted to take on the Mosul operation as that they are fighting across a dozen battlefields right now, mainly in Shia-dominated parts of Iraq.

Nor would the Popular Mobilisation units necessarily be welcomed in Mosul city, even by anti-ISIL militants.

Though Popular Mobilisation units have supported Sunni tribes in Ramadi, Dhuluiya and Heet, the situation in Mosul may be different. ISIL succeeded in seizing Mosul partly because of local resentment against the Shia-led security forces.

Moslawis are likely to react negatively to dominance by any major outside security force, whether Shia militias, Peshmerga or even federal army forces. In 2003 when the Saddam forces collapsed, Mosul immediately became a free-for-all where pop-up militant groups vied for dominance.

In all likelihood the full commencement of the battle of Mosul will need to wait until the summer of 2015 at the earliest.

Two risks will drive decision-makers in Baghdad, Washington and Erbil to hold back from assaulting the city.

The first is the risk of catastrophic failure; the bloody repulse of a hasty attack on the city, which could negatively affect Iraqi security force morale elsewhere and transfer the initiative back to ISIL.

But a second, equally serious risk is that of catastrophic success; that ISIL control could "pop" surprisingly quickly, creating a chaotic scramble for power in Iraq's second city between the Iraqi government, the Kurds, local Sunni militias and ISIL diehards.

If such an outcome can be avoided through the patient creation of a "day after" plan agreed upon by all the attacking forces, then the eviction of ISIL from Mosul might qualify as a "liberation" instead of just the commencement of a new chapter of fighting in that embattled city.

Michael Knights is the Lafer Fellow at the Washington Institute for Near East Policy. He specialises in the politics and security of Iraq. He has worked in every Iraqi province and most of the country's hundred districts, including periods embedded with Iraq's security forces.

The views expressed in this article are the author's own and do not necessarily reflect Al Jazeera's editorial policy.