WASHINGTON — As an alliance of Iraqi and Kurdish forces pushes to retake the city of Mosul from the Islamic State, 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 Islamic State’s leader in 2010 — declared a caliphate from the pulpit of an iconic 12th-century mosque.  If the Islamic State 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 by an American airstrike in August. Mr. 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 Islamic State 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 Islamic State’s predecessor, 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 jihadists’ six-year campaign depleted and fragmented Iraqi Sunni groups, making it easier for the Islamic State to control the Sunni heartlands when it returned in 2013.  “Historical events show that the mujahedeen of the Islamic State 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.  The Islamic State also released a video in August reportedly filmed in Wilayat al-Furat, one of the caliphate’s self-declared provinces. Unlike most Islamic State 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, Islamic State fighters attack and seize a camp they claim was secured by American and Iraqi government forces.  Amaq, the Islamic State’s news agency, 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 Mr. Adnani’s speech and articles about inhiyaz, are meant to prepare Islamic State fighters for the loss of territory. But the United States and its allies should pay attention, too.  For the Islamic State, 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 hide-outs for senior members — if they are not there already.  Iraqi officials already see signs of what the Islamic State’s 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 the Islamic State was returning to areas liberated since December 2014, recruiting new members and organizing hit-and-run and suicide attacks in populated areas. As in Egypt’s Sinai Peninsula and Pakistan’s rural northwest, overstretched security forces are not able to keep up.  As the Islamic State’s 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 the group to replenish their 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 Islamic State’s rebuilding. Iraq now 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 the Islamic State. The conflict in Syria further complicates the situation: Even if the Islamic State is driven from populated areas in both countries, the open desert border between them will make the group hard to chase.  The war against the Islamic State is unwinnable without filling the political and security vacuum that now exists in too much of Iraq. The Islamic State’s eventual retreat from Mosul will be a much-needed victory for the country. But unless the government in Baghdad enables Iraqi Sunnis to fill that void, it 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.”  A version of this op-ed appears in print on October 24, 2016, on page A21 of the New York edition with the headline: ISIS After Mosul. 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The group will retreat into the desert, just as it did in 2007. But can we stop it from re-emerging?