WASHINGTON • As an alliance of Iraqi and Kurdish forces pushes to retake the city of Mosul from the Islamic State in Iraq and Syria (ISIS), there should be no doubt about what the group plans to do next. It will fight to the bitter end to defend its most populous and symbolic stronghold.
After all, it was in Mosul that Abu Bakr al-Baghdadi - the city's leader for two years before he became the leader of ISIS in 2010 - declared a caliphate from the pulpit of an iconic 12th-century mosque.
If ISIS loses Mosul, the group has a clearly articulated contingency plan, a strategy it has frequently broadcast on multiple platforms for the past five months: inhiyaz, or temporary retreat, into the desert.
The word "inhiyaz" appeared in May, in the last speech delivered by Abu Muhammad al-Adnani, the group's spokesman who was killed in an American air strike in August. Adnani explained that territorial losses did not mean defeat and that militants would fight until the end and then retreat to the desert, preparing for a comeback, just as they did between 2007 and 2013.
Various ISIS outlets picked up the theme. Al-Naba, the group's newsletter, ran an article about the subject in August, recalling how the militants of the Islamic State of Iraq, the predecessor of ISIS, survived after they were driven out of Iraqi cities following the 2007 American troop surge and the tribal insurrection known as the Awakening.
While most militants retreated, according to the article, dozens of operatives remained to foment a terror campaign. The article explains - rightly - that the militants' six-year campaign depleted and fragmented Iraqi Sunni groups, making it easier for ISIS to control the Sunni heartland when it returned in 2013.
The war against ISIS is unwinnable without filling the political and security vacuum that now exists in too much of Iraq.
"Historical events show that the mujahideen of (ISIS) prevented the apostates from enjoying a single day of safety and security," the article says, in an ominous warning of what might be next for Iraq.
ISIS also released a video in August reportedly filmed in Wilayat al-Furat, one of the caliphate's self-declared provinces. Unlike most ISIS videos, which show urban fighting, this one featured desert combat, with footage of clashes near Rutbah, a strategic town in western Iraq on the road that connects Baghdad to Amman, Jordan. In the video, ISIS fighters attack and seize a camp they claim was secured by American and Iraqi government forces.
ISIS news agency Amaq distributed an excerpt from the video with English subtitles. Two weeks later, the same outlet reproduced and distributed a similar video, also filmed in the desert, concluding with a scene in which an Iraqi soldier's body is dragged down a street.
These videos, like Adnani's speech and articles about inhiyaz, are meant to prepare ISIS fighters for the loss of territory. But the United States and its allies should pay attention, too.
For ISIS, Wilayat al-Furat is no less important than Mosul. For long-term survival, the desert matters as much as the cities. Wilayat al-Furat is the only province that crosses the Iraq-Syria border, and the territory and remote areas like it are potential hideouts for senior members - if they are not there already.
Iraqi officials already see signs of what the ISIS retreat into the desert could mean. Two security officials in Salah ad Din, a province north of Baghdad, said in a recent TV interview that ISIS was returning to areas liberated since December 2014, recruiting new members and organising hit-and-run and suicide attacks in populated areas. As in Egypt's Sinai Peninsula and Pakistan's rural north-west, overstretched security forces are not able to keep up.
As the ISIS leaders remember, this is what happened after 2007. The desert became a base, mostly for foreign fighters, while Iraqis stayed behind. The group's presence in rural areas also allowed it to replenish its coffers with highway robbery and extortion. The militants focused their attacks on the tribal adversaries and Iraqi security forces, sowing distrust and fear, making conditions ripe for their return six years later.
But this time, conditions are even more conducive to the rebuilding of ISIS. Iraq is more politically and socially fractured than it was then. And, as one Iraqi who participated in the Awakening Councils told me, there is now no Sunni group in Iraq that can fill the void left by ISIS.
The conflict in Syria further complicates the situation: Even if ISIS is driven from populated areas in both countries, the open desert border between them will make the group hard to chase.
The war against ISIS is unwinnable without filling the political and security vacuum that now exists in too much of Iraq. The eventual retreat of ISIS from Mosul will be a much-needed victory for Iraq. But unless the government in Baghdad enables Iraqi Sunnis to fill that void, ISIS will once again emerge from the desert.
•Hassan Hassan is a resident fellow at the Tahrir Institute for Middle East Policy and an author of ISIS: Inside The Army Of Terror.