As the Islamic State group loses its remaining strongholds in Iraq and Syria, Defense Secretary Jim Mattis is facing a growing chorus of questions from NATO allies and partners about what the next steps will be in the region to preserve peace and ensure the militants don’t rise again.

Heading into a week of meetings with Nordic countries and allies across Europe, Mattis must begin to articulate what has been a murky American policy on how the future of Syria unfolds.

Speaking to reporters traveling with him to Finland, Mattis said the main question from US allies is: what comes next? And he said the key is to get the peace process on track.

“We’re trying to get this into the diplomatic mode so we can get things sorted out,” said Mattis, who will meet with NATO defense ministers later this week. “and make certain (that) minorities — whoever they are — are not just subject to more of what we’ve seen” under Syrian President Bashar Assad until now.

US Secretary of State Rex Tillerson in late October repeated Washington’s call for Assad to surrender control, looking past recent battlefield gains by his Russian-backed forces to insist that “the reign of the Assad family is coming to an end.”

Tillerson made the comments after meeting with the UN’s envoy for Syria, Staffan de Mistura, who later announced plans to resume UN-mediated peace talks Nov. 28. It will be the eighth such round under his mediation in Geneva since early 2016.

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Mattis said intelligence assessments two to three months ago made it clear that the Islamic State group was “going down.” He said information based on the number of IS individuals taken prisoner and the number of fighters who were getting wounded or were deserting the group made it clear that “the whole bottom was dropping out.”

But while he said the effort now is to get the diplomatic process shifted to Geneva and the United Nations, he offered few details that suggest the effort is moving forward.

In addition to the diplomatic efforts, Mattis said the US is still working to resolve conflicts with Russia in the increasingly crowded skies over the Iraq and Syria border, where a lot of the fighting has shifted.

On Nov. 3, Assad’s military announced the capture of the eastern Syrian city of Deir ez-Zour, while Iraqi Prime Minister Haider al-Abadi proclaimed victory in retaking the town of Qaim on the border, the militants’ last significant urban area in Iraq.

Focus has now turned to Boukamal, the last urban center for the militants in both Iraq and Syria where Syrian troops —backed by Russia and Iranian-supported militias — and US-backed Syrian Democratic Forces are vying for control of the strategic border town.

The proximity of forces in the area has raised concerns about potential clashes between them as they approach Boukamal from opposite sides of the Euphrates River, and now from across the border with Iraq.

Mattis said that as forces close in, the fighting is getting “much more complex,” and there is a lot of effort on settling air space issues with the Russians.

He also declined to say whether the US will begin to take back weapons provided to Syrian Kurdish fighters, known as the YPG. The US has argued that the YPG has been the most effective fighting group in the battle to oust IS from Raqqa, but Turkey opposed the arming effort because it believes the YPG is linked to a militant group in Turkey.

The US has pledged to carefully monitor the weapons, to insure that they don’t make their way to the hands of insurgents in Turkey, known as the PKK. The US also considers the PKK a terrorist organization, and has vowed it would never provide weapons to that group.

Turkish officials have said that Mattis reassured them by letter that arms given to the Syrian Kurds would be taken back and that the US would provide Turkey with a regular list of arms given to the fighters.

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While in Finland, Mattis will attend a meeting of a dozen northern European nations, which are primarily concerned about threats from Russia.

“They are focused on the north,” said Mattis, adding that he plans to listen to their thoughts on the region and determine how the US can help, including what types of training America could provide.

“It is an opportunity to reiterate where we stand by our friends,” said Mattis, “if any nation, including Russia, seeks to undermine the rules of international order.”

During a press briefing later on Nov. 6, Denmark Defense Minister Claus Hjord Frederiksen told reporters that allies must continue to be present in the region because of the risk that IS would rise again.

“We’re not so naive that we think that terrorism is removed from this earth, but of course it is very important to have taken geographical areas from them so they can’t attack or rob or whatever — using the income from oil production to finance their activities,” said Frederiksen. “We foresee therefore years ahead we will have to secure that they cannot gain new ground there.”

An ISIS expert claims there is a glaring “Achilles heel” present in the US strategy in Iraq and Syria, stating that the lack of any planning for the political future of the region after the terrorist group is wiped out will nullify the military gains made against the group.

And while the fall of ISIS’s de facto capital of Raqqa, Syria might mark a significant gain against the terrorist group also known as the Islamic State, there is much work left to be done.

“Only a fool would call this a victory,” Hassan Hassan, a senior fellow at the Tahrir Institute for Middle East Policy and the co-author of “ISIS: Inside the Army of Terror,” told The New Yorker. “It’s only the expulsion of ISIS fighters from a wasteland. It’s not a victory, not only because of the destruction. It’s also not a victory because there’s a shameless lack of a political track to supplement the military track. That’s the Achilles heel of Operation Inherent Resolve. They don’t have a political vision about what will happen after ISIS.”

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The destruction Hassan mentions is almost total in Raqqa. The activist journalism group Raqqa Is Being Slaughtered Silently claims that 90% of the city has been destroyed by the months of fighting between ISIS, the Kurdish-led Syrian Democratic Forces, and the US coalition.

The group has documented more than 3,829 airstrikes and 1,873 civilian deaths throughout the urban battle, and says 450,000 people remain displaced from the city.

Yet Hassan’s main argument is that the main threat to the success of the US-led mission is that there is no political plan for what will come after ISIS’s territorial defeat.

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Professor Robert Pape, the director of the Chicago Project on Security and Terrorism at the University of Chicago, said he agrees.

“When we invaded and conquered Iraq in 2003 we created ungoverned space for Sunni Arabs in Iraq which then spilled over in nearby Syria,” Pape says. “The worry here is that as that area of Iraq and Syria now could remain ungoverned space from the perspective of the Sunni Arabs, this problem may just simply fester and continue.”

ISIS, and the war to defeat it, has inflicted enormous violence upon the Sunni Arabs of the region, and its effects will stick with the Sunni populations of Iraq and Syria for generations.

And throughout the campaign to liberate Sunni regions previously under the the rule of ISIS, Iraq has employed Shiite militias with ties to Iran, called the Popular Mobilization Forces, which have been suspicious of Sunni villagers in conquered ISIS territory. Iraq’s own security forces have also frequently resorted to brutality against civilians in places like Mosul, which was an ISIS stronghold until recently.

Meanwhile, vast swaths of eastern Syria remain controlled by Kurdish-led militias in the form of the Syrian Democratic Forces, or by the Shiite-led Syrian government.

An additional yet significant ethnic challenge lies in how to divide power between Kurds, Shiites, and Sunnis in Syria and Iraq after the dust settles. Already, Iraq’s central government is asserting itself in regions controlled by Kurds around Kirkuk and Mosul, where clashes have occurred.

Such post-conflict realities in the Sunni regions of Iraq and Syria have led to widespread distrust between locals and the governments and militaries that now control them and have deepened the same feelings of political isolation among Sunnis that led to the rise of ISIS between 2007 and 2013.

According to Hassan, the “Achilles heel” of the US-led coalition’s strategy is that it makes no preparations to resolve these complex problems, and focuses solely on a military victory over ISIS. In his view, such a limited approach will only hasten the return of another Sunni insurgent movement in the region.