More than two years after the fall of the Mosul, to the then expanding Islamic State of Iraq and Syria (ISIS), thousands of soldiers are now rushing to capture the city in northern Iraq. As prominent Shiite cleric and political figure Muqtada Sadr has said, Mosul is a battle of national destiny.

In June 2014, the capture of Mosul laid the groundwork for IS's expansion and its liberation may well mark the beginning of the group's downfall. Yet the way the battle will be fought is as crucial as the success of the operation itself. Whether the future of Iraq is one of unity and reconstruction or just another chapter in the country's history of instability depends on the battle that just started.

The beginning of the operation comes less than 24 hours after IS lost the symbolic town of Dabiq in northern Syria to a coalition of Syrian opposition fighters backed by Turkey. The timing is not coincidental, as the US anti-ISIS coalition and its allies understand that IS is most vulnerable when faced with multi-pronged offensives.

Dabiq is also an important symbolic prize. It is set to be the site of an apocalyptic battle between the faithful and the Crusaders, according to IS propaganda based on a Hadith. With the capture of Dabiq, however, it seems the final battle has been postponed.

The symbolic value of the city and its impact on IS morale is in fact the whole point. The group has been steadily retreating, offering little resistance since the battle for Manbij, even in a such symbolic city as Dabiq.

Anti-ISIS forces likely bet that the steady retreat – thus far looking more like an organised withdrawal – will turns into a full on debacle. This is particularly important when it comes to Mosul, the latest in a series of battle pitting a diverse mosaic of pro-government forces against the jihadist group in Iraq.

Battle for Mosul
Iraqi forces are pictured cheering as they gather at Qayyarah military base, about 60 kilometres (35 miles) south of Mosul Ahmad al-Rubaye/AFP

So as not to stir sectarian tensions, pro-government Shiite militias as well as the Kurdish Peshmerga are set to remain outside of the city, leading the first assault to reach the outskirts of Mosul but letting other regular forces enter the city. This strategy was relatively successful in Fallujah and Ramadi, although it did trigger a mass exodus of locals in the latter.

Yet Mosul isn't Ramadi nor Fallujah. The city is far bigger and IS has had time to set up its defenses, undisturbed by Iraqi or Kurdish forces. An estimated 1.5 million civilians are still living in the city where Abu Bakr al-Baghdadi declared the establishment of a so-called "caliphate" two years ago.

Efforts to limit civilian casualties and preserve the fragile ethnic and religious stability of Iraq means a significant portion of the 80,000 anti-IS fighters participating in the operation may actually not enter the city. This includes the 15,000-18,000 Kurds as well as the Popular Mobilization Units (a coalition of mostly Shiite militias) and other smaller militias.

The rest of the anti-ISIS force, an estimated 45,000 soldiers should be enough to capture Mosul. Yet whether they will manage to do so swiftly enough to minimise civilian casualties, should most of IS's force remain in the city, is unclear .

This is why, as they march toward the city, the anti-IS force must stage their own "shock and awe" offensive to break IS lines of defense, and more importantly its morale. The first days of the battle will be critical, as a rapid advance will encourage rebellion against the group and fuel inner dissension within its ranks.

But the real danger is that rivalry between the Kurds, Shia, central government and Turkish-backed forces will waste one of the best shots Iraq has had at coming together.

After all, IS spokesperson Abu Mohammed al-Adnani himself stated that the group was ready to "return to the desert" – Iraqi forces wouldn't mind if they do so now. Such a victory would also counter-balance the perception that Iraqi forces did not fight to defend the city in 2014, and reestablish their prestige as the national force Iraqis can be proud of.

Less than a day after Prime Minister Haider al-Abadi announced that the "campaign to liberate Mosul has begun" and promised "the Iraqi nation will celebrate victory as one", Iraqi forces have made significant progress. This offensive reflects their goal of leaving a corridor for IS to withdraw, while swiftly and decisively approaching the city. Yet, despite echoes of infighting and rebellion within the city, which are difficult to confirm, on the front line IS has not faltered. More than 10 car bombs have exploded as anti-IS forces led their first assault.

Should the initial offensive not convince IS militants to scrap their plan for a last stand in Mosul, the battle could turn into a slow and costly urban war. The UN Office for the Coordination of Humanitarian Affairs (OCHA) already expressed concerns other the unprecedented number of residents the offensive displace.

According to a statement by the organisation, more than 200,000 residents of Mosul may flee the city in the first week of the operation, and in a "worst-case scenario, one million people could be displaced and 700,000 could need emergency accommodation"... Whether the various international and local humanitarian organisation can cope with such a crisis is doubtful.

On the other hand, while Abadi promised the Mosul victory would be celebrated as one, a protracted conflict may well reveal the deep divisions behind today's admirable show of unity.

The presence of Turkish-backed forces and possibility of Turkish ambition to participate in the offensive already prompted a harsh response from the central government in Iraq. Turkey plays on fear that the Shia militia will, sooner or later, enter the city and may exact revenge against the local population.These fears are largely exaggerated but not completely unfounded.

But the real danger is that, as the various group progress in the city, the rivalry between the Kurds, Shia, central government and Turkish-backed forces will brush aside the current show of unity and waste one of the best shots Iraq has had at coming together. This unity will be tested, as IS will likely return to clandestine guerrilla warfare in Iraq, while planning the defence of the rest of its crumbling "Caliphate".

Michael Horowitz is director of intelligence at Prime Source, a Middle-East-based geopolitical consultancy producing updates and analysis on the Syrian and Iraqi conflicts.