The military offensive against Isis forces in Tikrit, Iraq, is shaping up to be one of the most decisive moments in the profound struggle unfolding across the region. On Saturday no less a figure than General Martin Dempsey, chairman of the US joint chiefs of staff and so America’s top officer, declared that the combined force of Iranian-backed militias and Iraqi government troops was likely to prevail against Islamic State forces in the battle and that the Islamist militants would be pushed out of Saddam Hussein’s home town. “The numbers are overwhelming,” he said.

But there are ominous signs that the campaign faces many perils and there are fears that its impact may unleash fresh waves of sectarian conflict, as well as long-term rebalancing of political forces in the region.

The campaign, which entered it’s second week on Sunday, is the first serious attempt to dislodge Isis from a Sunni area it has governed since the group’s military blitz in Iraq last June. Despite the American-led air strikes since the summer, the militant group has faced little pressure inside what can be described as its heartlands, such as Mosul, Falluja, Raqqa and Deir Ezzor. The offensive in Tikrit is therefore a critical development that will be monitored closely and nervously by almost everyone involved in the conflict. It is also the first major effort led by pro-government forces without consulting the United States and members of the international coalition. That latter fact leaves many question marks about the campaign. The Iraqi government portrays it as a national effort, led by the security forces and including thousands of Sunni tribal fighters. It also claims that Tikrit is all but empty of civilians.

But these claims are not entirely accurate. Hashd al-Shaabi, the umbrella organisation for Iranian-backed Shia militias, put together in the wake of Isis’s takeover of Mosul in June to serve as a de facto replacement for the army in the fight against the terror group, is leading the offensive. Any Sunni forces participating, notwithstanding their numbers, take a back seat at best.

The claims with regard to civilians are particularly alarming. Hashd al-Shaabi has a track record of human rights abuses and sectarian and ethnic cleansing, as documented by Human Rights Watch. Instead of highlighting that Tikrit’s civilians have almost completely fled the city, the government should focus on ensuring no similar reprisals against civilians by these notorious militias are committed in Tikrit.

A video circulated over the past week purportedly showed pro-government forces killing a boy. Many fear such acts will take place on a larger scale during the offensive, which is likely to take weeks, if not months. These fears are heightened by two main things promoted by Hashd al-Shaabi. First, the Tikrit offensive has been portrayed by many Shia militias as revenge for the Camp Speicher massacre committed last summer, when Isis killed at least 1,566 Iraqi air force cadets as government forces attempted to retake Tikrit from the group. Second, Shia militias have been warning they would reserve special ire for Sunni “collaborators” – the type of language Iraqis of all sects and ethnicities have grown to identify as having cleansing connotations.

The aim of the campaign seems to be more political than tactically sound. A victory in Tikrit will boost Hashd al-Shaabi immensely in Iraq, politically, militarily and financially. The likely motivation is that Hashd al-Shaabi recognises that the fight against Isis in the Sunni districts of Tikrit will be a massive political win. Over the past few months, Hashd al-Shaabi has been the centre of debate in Iraq on a social and political level. The militias that constitute the core of the organisation pushed for its integration within the national institutions and opposed the formation of a “national guard” that would include the arming and funding of Sunni tribal fighters.

Also, many Hashd al-Shaabi fighters have started to lose the appetite to fight beyond their areas, either because they have not been paid for months or because they see less need to mobilise nationally against Isis, thanks to the western air strikes and the apparent demarcation of borders of Kurdish, Isis and Shia militias over the past few weeks, as Isis is largely contained within Sunni areas. It is important to remember that many Shia do not see it as their job to liberate Sunni areas from Isis.

The fight against Isis in Tikrit is likely to drag on and backfire. If Isis proves to be entrenched in Tikrit and capable of holding its ground, that may push Shia rank and file to question the logic of fighting in Tikrit, especially if the number of casualties grows. The expectation is that the fight will be short and decisive, which is not likely.

A slowdown in the fight might also drag the international coalition into the conflict, which is probably the calculation of Hashd al-Shaabi and its Iranian backers. Since it is clear to those on the ground that Hashd al-Shaabi started the offensive, the Iranian-backed organisation can still claim credit for any victory. The calculation is that Washington would be desperate for such a huge win against Isis before launching an offensive against it in Mosul, so the US would still provide the air cover without getting credit for the initiative.

Regardless of who wins the fight, appearances matter. For some Sunni in Iraq and outside, the fact that Iranian generals are leading a Shia-dominated offensive in the Sunni birthplace of Saddam Hussein, and without consulting the Americans, is enormously evocative. Dislodging Isis from Shia areas or from mixed areas by Shia militias is understandable. However, unilaterally launching a revenge-tinged campaign in a Sunni area by sectarian militias with a track record of acts of cleansing will unavoidably be seen as a vigilante operation.

If the history of fighting Isis, and its previous incarnations, can teach us one lesson, it is that Isis can only be defeated by Sunni from within. After the invasion of Iraq in 2003, Falluja, for example, was flattened at least twice to defeat the Sunni insurgents there, but Isis today reigns supreme in the city.

The idea that the offensive in Tikrit is a national effort is a myth. While the international coalition might be tempted to see Isis finally being defeated in one of its strongholds, it will do well to avoid taking what seems to be the easy route.

Isis will benefit from the offensive in Tikrit, even if it loses militarily, as long as the victors are sectarian militias that behave in a similar manner. To defeat Isis in Tikrit is hard, but not impossible. The most likely outcome is that the city will be destroyed. But the defeat of Isis in Tikrit will not help in the fight in Mosul. On the contrary, it will convince Sunni communities living under Isis that the alternative is just as bad.

Hassan Hassan is a Middle East analyst and co-author of the bestseller, Isis: Inside the Army of Terror. Follow him on Twitter: hxhassan