The war against ISIS has reached a crucial point

Iraq Iraq Paramilitary Fighters Shi'ite Islamic State ISIS Flag TikritREUTERS/Alaa Al-MarjaniIraqi security forces and Shi’ite paramilitary fighters hold an Islamist State flag, which they pulled down in Tikrit, March 31, 2015.

Anti-ISIS ground forces scored their biggest victory against ISIS last week, retaking most of central Tikrit.

The offensive marked the first time ground forces had recaptured a Sunni city from the incredibly brutal terrorist group that has nevertheless positioned itself as a vanguard for Iraq’s minority religious sect.

The anti-ISIS ground force — which consisted mostly of Shi’ite militia groups fighting alongside Iraq’s largely Shi’ite uniformed military and Iranian Revolutionary Guards — has not yet regained the strategic initiative from their jihadist opponents. ISIS still rules over much of Iraq and Syria. It’s grip over major cities like Raqqa and Mosul hasn’t been threatened. The group just launched a fresh wave of carbombs in Iraq and gained a foothold in the Yarmouk Palestinian refugee camp near Damascus.

Tikrit also shouldn’t inspire much confidence about how the rest of the ground war against ISIS might go.

It took a reported 20,000 troops over a month to defeat ISIS in Tikrit, an effort that required continuous shelling of the city and inflicted heavy causalities on anti-ISIS ground forces. Government-allied militants falsely claimed to have retaken most of the city in mid-March, while Sunni involvement in the effort to liberate the city was fairly minimal.

As the New York Times reported on April 7th, American and Iraqi officials are trying to formulate a new post-Tikrit military template in which there’s less reliance on sectarian militia groups and much greater Sunni military involvement.

The heavily qualified success in Tikrit — where it eventually took a sectarian force with US air support weeks to expel ISIS — raises the question of what kind of success the ground forces can really hope to make in the near-term. It’s obvious that the assault on Mosul that US Central Command announced in February isn’t going to happen during the spring. And just after the announcement, military planners told the Daily Beast that the spring target for a Mosul offensive wasn’t realistic anyway.

But Mosul’s still the next big challenge, the second-biggest city in Iraq and one of ISIS’s biggest pieces of evidence that it actually rules over anything even resembling a “caliphate.” Victory in Mosul would turn the tide of the war, deflating ISIS’s prestige and winning over Iraqi Sunnis who don’t actively support the group but have still looked to it as their sect’s strongest military remaining military force.

And less than two months ago, US military planners believed that Mosul could realistically be retaken by the existing ground-forces.

Ali Khadery, the CEO of Dubai-based Dragoman Partners and a former advisor to US Central Command with extensive on-the-ground experience in Iraq, doesn’t think that’s possible. He spoke to Business Insider prior to the conclusion of major fighting in Tirkit, and said that the Tikrit campaign had gone so poorly that it wiped out much of the short-term hope for a successful assault on Mosul, which is Iraq’s largest majority-Sunni city.

“If they can’t take Tikrit with relative ease despite outnumbering the insurgents by 20-40:1 and having a sovereign power, Iran, behind them, I don’t know how they’re going to take a city that is at least 10 times as large, both geographically and demographically,” Khadery told Business Insider.

ISIS Mosul IraqAPFleeing Iraqi citizens from Mosul and other northern towns wait in a long traffic queue at a Kurdish security forces checkpoint at a highway between the Iraqi city of Mosul and the Kurdish city of Irbil.

There’s an even more fundamental problem. Tikrit’s size and symbolism as the birthplace of Saddam Hussein, who invaded Iran in 1980 and slaughtered thousands of Shi’ites in the wake of the 1991 Gulf War, made it an attractive and manageable target for Iran and its Shi’ite militant allies.

Mosul is just a very large and incredibly hostile city, and it’s debatable whether Shi’ite sectarian forces would be willing to lose thousands of soldiers retaking it in the name of a united, multi-confessional Iraq.

The Kurds, meanwhile, have little interest in retaking Mosul, since their unresolved territorial claims in northern Iraq mostly regarded Kirkuk — a city the Peshmerga moved into last June as ISIS blitzed across northern Iraq.

“Given that Mosul has been the seat of Sunni Arab power in Iraq for centuries, the coming months will either foreshadow reconciliation or a new chapter in the broader regional holy war,” Khadery told Business Insider.

But there’s reason to believe that an offensive on Mosul could make some crucial headway against ISIS so long as its aims remain tactical and limited, at least at first.

Michael Pregent, a former US Army intelligence officer who was based in Mosul during the “surge” period of the US campaign, laid out a plausible roadmap for moving into the city during a conversation with Business Insider prior to Tikrit’s recapture. He said that ground forces could score a significant victory by focusing on a few vital pieces of infrastructure and resisting the urge the immediately retake the more radicalized and dangerous western half of town.

“They don’t have to take the whole thing,” says Pregent. “They just to be able to take the airfield in Mosul or key bridges between east and west Mosul. That would be enough to deconstruct the narrative of the capital of the caliphate in Iraq as being under the control of ISIS … if you’re able to bomb ISIS targets from an airfield in the isis capital of Iraq, it’s a very big deal.”

As Pregent notes, the airport lies at the southern edge of the city and can be retaken without having to move into heavily populated areas. There are also only 5 bridges across the Tigris river in central Mosul; ground forces would only have to retake one side of them to cut off ISIS supply lines and turn the momentum of the battle.

MosulGoogle Maps screenshotThe position of the Mosul airport.

Pregent adds that the US military was able to maintain and defend a key base inside the Mosul airport despite western Mosul being a hive of insurgent activity. The US got around this challenge by picking its battles in the city wisely: “I would argue that we never secured Mosul,” he explained. “We just left parts we thought were too difficult to handle alone.”

The war against ISIS in Iraq will inevitably turn on Mosul. It’s the country’s second-largest city, and ISIS’s most substantial claim to legitimacy. If the momentum swings, it will most likely swing in Mosul.

At the same time, it might not be possible to take the entirety of the city in a single push. And the sectarian victory in Tikrit could only convince Sunnis to aid in the city’s defence against a force consisting of soldiers from Shi’ite militias, the Baghdad government, and Iran.

“This is an urban Sunni population that believes it doesn’t have anywhere to turn,” Pregent explained. “I don’t think they turn towards ISIS. I just think they’re willing to fight for their pride.”

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