The US plan to kick ISIS out of its most important city in Iraq, explained

The US has announced a plan for Iraqi troops to invade Mosul, Iraq's second-largest city, which has been under ISIS control since June 2014. The offensive, planned for April or May with US support, is potentially risky, but could be a turning point in the war against ISIS.

The success of the Mosul campaign is critical to the future of the war against ISIS — and also to the future of the Iraqi state and its relationship with the US. But there may be more going on here than meets the eye.

Why Mosul is so important

Refugees forced out of Mosul by ISIS's advance. (Emrah Yorulmaz/Anadolu Agency/Getty Images)

Mosul is the crown jewel of ISIS's Iraqi territory and by far the largest city under the group's control. Losing it would be a devastating blow to ISIS's position in the country.

Mosul is a center of Sunni life in the country. Over one million people are thought to live there, making it potentially a sixth of the total population under ISIS control. It's also at a critical location in northern Iraq, providing ISIS a base from which to threaten both Iraqi Kurds to the north and other Iraqi cities, such as Tikrit, Samarra, and even Baghdad south along the Tigris River.

Taking back Mosul, then, would remove ISIS from its core base in northern Iraq, making it very hard for it to hold territory further south. It would no longer be able to fundraise by running extortion rackets in the populous city. Without Mosul, ISIS likely could not accomplish its core objective of building a real state on Iraqi land.

The thing that makes ISIS different from other violent Islamist groups, like al-Qaeda, is that it has tied the identity and message to its ability to control territory. ISIS's primary short term objective isn't to kill Iraqis or Americans; it is to to build a new government founded on what it sees as the ideal of the original Islamic caliphates. Without Mosul and the surrounding environs, its so-called Islamic State would be shunted to less important parts of the country. If it can't hold Mosul, ISIS will have failed at its core objective in Iraq.

"The Iraqi government thinks this is cutting off the head of the snake; that if they can take back Mosul, they will have struck a death blow to ISIS in Iraq," Michael Knights, the the Lafer Fellow at the Washington Institute for Near East Policy, says.

Liberating Mosul could "turn ISIS back into a more normal insurgent movement that fights from the shadows rather than controls major cities," Knights says. That'd be devastating for ISIS's recruitment and retention: their ideological appeal is linked to the idea that they control an actual caliphate. Losing Mosul would be a crushing blow to that idea, hurting ISIS's ability to attract fighters around the world to its army in Iraq and Syria. The stakes are high.

How the plan works

Iraqi troops in training. (Mohammed Sawaf/AFP/Getty Images)

The plan for Mosul, as described by US Central Command (CENTCOM), is an audacious test of the American strategy for combating ISIS. It's far from clear that it will actually succeed in clearing ISIS out.

According to CENTCOM, the offensive will launch in April or May. A joint force of 25,000 Iraqi soldiers and Kurdish fighters will lead the assult. That includes five regular Iraqi army brigades, three Kurdish brigades, three Iraqi reserves, and some specialized Iraqi counterterrorism forces. Critically, there will also be a local police-tribal force designed to hold the territory after it's cleared. All of this will be supported by American airpower and possibly other forms of support such as weapons shipments.

They'll be facing a dug-in ISIS force. The US estimates there are likely 1,000 to 2,000 ISIS fighters in the city now, but the group currently numbers an estimated 20,000 to 31,500 in total according to the CIA (some private analyst estimates go as high as 100,000). So it could certainly shift more fighters to Mosul in anticipation of the assault.

Knights suggests that, one the fight begins, there will likely be 1,000 to 2,000 "core guys who will fight to death" from ISIS there. Less committed ISIS-aligned fighters might bring the group's Mosul force to anywhere between 3,000 and 10,000; 5,000 is Knights' best guess. So the ISIS force will be substantial, but almost certainly much smaller than the Iraqi force.

"You've got that many guys, in a large city," Knights says, and "that creates a massive, massive military task." ISIS fighters can seed the city with improvised explosive devices, essentially creating minefields to be cleared. Iraqi cities, certainly, are no stranger to the danger and resilience of an entrenched insurgency.

"There are bombs to be defused, city blocks to be cleared of enemy, houses to be searched for weapons caches," Knights explains. He estimates doing that would require 40,000 Iraqi soldiers. "There has to be a factor we don't understand to make 25,000 work," he concludes.

Moreover, it's not clear that Iraqi troops are well-trained enough to perform all of these tasks. The US has only trained a few thousand Iraqi soldiers so far, so there are serious doubts as to whether enough of them will be ready for the Mosul offensive by the April/May target date.

The plan has more to do with Iraqi politics than meets the eye

Iraqi Prime Minister Haider al-Abadi speaks at a meeting at the Waldorf Astoria hotel on September 25, 2014 in New York City. (Spencer Platt/Getty Images)

It was probably just a matter of time until the US and Iraq moved to retake Mosul, but the decision to announce the assault in advance has understandably raised eyebrows — including in Baghdad. The announcement may in fact be more about diplomacy between the US and Iraq than about military strategy, and the invasion may not take place until much later in the year.

"I don't know where the American official got this information ... they absolutely do not have knowledge on this issue," Iraqi Defense Minister Khaled al-Obeidi said to Reuters of the announced timetable.

The skepticism from the defense establishment diverges sharply from what you hear from some Iraqi political leaders; Prime Minister Haider al-Abadi said in mid-February that Mosul would be taken back in several moths. Iraq analysts believe the US announcement may have been intended to placate Abadi and others, and that the specific timetable may be a bit of a smokescreen.

"The United States' real target was the Iraqi political class," Iraq analyst Joel Wing writes at his blog on Iraqi issues. CENTCOM wants "to show them that Washington has a plan to defeat the insurgency."

The US might also conceivably be sending a message to Mosul's residents, who are tired of living under ISIS's boot. "There's reasons why we don't want people in Mosul to think liberation is so far away," Knights explains. That could have to do with "efforts to build allies in the city and perhaps even foment an uprising."

These political motivations suggest that it'd be wrong to treat the April/May deadline as hard and fast. "May is just an initial goal," a CENTCOM spokesperson told the Telegraph. "If senior leaders do not feel that by then they are in a position where they can be successful, they can change it."

Wing suggests the end of 2015 is a more reasonable target. Knights thinks the fall is more realistic, but believes it's still possible that the Iraqis could chance something in April.

The Iraqi army will likely conquer Mosul eventually

Kurdish peshmerga near the Mosul Dam. (Ahmad al-Rubaye/AFP/Getty Images)

ISIS is in an extremely vulnerable position in Iraq; it's overstretched, friendless, and hasn't won a major battlefield victory in months.

"The Islamic State ... will lose its battle to hold territory in Iraq," former National Security Council director for Iraq Doug Ollivant writes in War on the Rocks. "The outcome in Iraq is now clear to most serious analysts."

So all of the back-and-forth between the Iraqis and the Americans right now isn't about whether they will eventually move on Mosul. Rather, the debate is over how best to plan a Mosul offensive in order to avoid failure on the first try, successfully hold the city after it's taken, and minimize casualties.

The key questions, Knights says, are "not if the Iraqi army can liberate Mosul, it's when, and at what cost."

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