Conventional military doctrine cautions against urban operations. Typically they require huge numbers of troops, are painstakingly slow and result in high levels of casualties particularly among the civilian population.
Yet, the Iraqi government is not fighting a conventional war and, as it prepares to wrest back control of Mosul from ISIL, it will be engaged in a battle where the tactics of each side will be determined by their long-term strategies.
Why is it important to retake Mosul from ISIL?
For the Iraqi security forces (ISF) not only do they need to dislodge a brutal and - at times - literally suicidal opponent, but they need to retake the city in a way that facilitates Baghdad's future rule over the Sunni minority population and the encroaching ambitions of the Kurdish Regional Government (KRG).
Whereas ISIL needs to secure the few advantages available in a battle it cannot win. This means ensuring the survival of key personnel while ceding control of a city in such a way that deters future attacks.

Assault on Mosul

The assault on Mosul is the latest - and biggest - stage in a three-part campaign put together by the Iraqi and US governments in the aftermath of the city's capture in June 2014.
OPINION: The day after the battle for Mosul
This has seen ISIL's advance stopped primarily through air strikes, the retraining and equipping of Iraq's security forces (ISF) and then finally their deployment, along with allied Shia militias, to sequentially take back lost ground.
Iraqi army gathers after the liberation of a village from ISIL, south of Mosul [Reuters]
To a large extent, the campaign has succeeded, and swaths of Iraq once under ISIL control, including former strongholds along the Euphrates river valley, are now back under the control of Baghdad.
Yet, it has also done little to ease sectarian tensions with widespread claims that Shia militias carried out atrocities during and after the recapture of key Sunni cities, such as Ramadi and Fallujah.
It is to prevent the reoccurrence of such incidents and to counter previous accusations of sectarian misrule that Baghdad has seemingly ordered the Shia Popular Mobilisation Forces not to enter Mosul itself.
A similar ruling also applies to the Kurdish Peshmerga, although this is driven primarily by concerns of creeping KRG influence over parts of northern Iraq. As such, Iraq's official security forces will have sole responsibility for the fighting in the city itself.
If Iraq's military are choosing their troop dispositions with an eye to governing Mosul, then for ISIL their tactical decisions are evidence of a bleaker future.

If Iraq's military are choosing their troop dispositions with an eye to governing Mosul, then for ISIL their tactical decisions are evidence of a bleaker future.
Given the forces arrayed against them and Baghdad's determination to seize back control of the city, ISIL knows it cannot hold Mosul indefinitely.
Yet with their long-term survival at stake, they both need to extract key personnel while still inflicting enough casualties on their opponents to propagate a moral victory and delay future encroachments on their remaining territories.

Tactical flexibility

To that end, it is likely that ISIL will combine the tactical flexibility and brutality that they have become renowned for.
Even though they have promised a fight to the death in the streets of Mosul, the need to preserve leadership and manpower means this honour is likely to fall predominately on foreign fighters - who are harder to exfiltrate from the city.
KRG: Mosul victory depends on ISIL fighters' morale
In addition to those fighters who remain in place, ISIL will also rely heavily on their technical expertise to inflict significant casualties on the attacking forces. This will include numerous Improvised Explosive Devices (IEDs), such as those initiated by pressure pads triggered by the advancing ISF, as well as innovations such as the exploding drone that was recently lethally used against Kurdish and French forces.
Similarly, given their effectiveness there is likely to be the widespread use of suicide attacks (according to 2015 US State Department figures, these attacks are 4.6 times as lethal as non-suicide attacks).
Typically, these are conducted both by those seeking martyrdom and other more vulnerable individuals co-opted into such attacks - such as children and the mentally ill.
Attacks in other areas of Iraq are also more likely as ISIL seeks to exploit the vulnerabilities offered by the redeployment of so many troops to the Mosul area. US officials have likened countering ISIL as to playing "whack-a-mole" in that they withdraw in front of an offensive only to pop up elsewhere.

Baghdad's impotence

ISIL will look to conduct such attacks both to highlight Baghdad's impotence to secure the territory under its control and to relieve some of the pressure on the defenders inside Mosul letting them extract more men and equipment before the real battle gets under way. This may have been the motive for this weekend's attack on Kirkuk during which ISIL attackers took over three districts and killed dozens of people.
Finally, as in the past when forced to give up territory, ISIL are likely to launch a wave of car bombings and suicide attacks in heavily populated Shia areas or the government district in Baghdad. Such attacks would generate huge amounts of publicity that would distract from the impending loss of Mosul, highlight weaknesses in the country's security systems and serve as a warning against further government offensives.
For ISIL, the loss of Mosul will pose as much of an existential threat as its fall did to the Iraqi government in July 2014. The difference being that ISIL are now surrounded on all sides and have no allies to come to their aid.
They know that once they have lost Mosul, their opponents will turn on their capital, Raqqa, and once that has fallen it will be the end. It is this that determines their choice of tactics now even if the battle is ultimately fruitless.
Crispian Cuss is a former British Army officer who lives and works in Southeast Asia. He currently acts as a defence and security consultant.
The views expressed in this article are the author's own and do not necessarily reflect Al Jazeera's editorial policy.