A US-led military alliance succeeded in eliminating ISIS in Iraq and Syria only last March. It is remarkable that only about 2,000 US troops took part in this effort, a fraction of those deployed in Iraq or Afghanistan at the height of those wars. . The key to success in Syria was that the United States worked side by side with local militias, namely the SDF, whose backbone was the YPG.

However, in one contact with Turkish President Recep Tayyip Erdogan, US President Donald Trump gave the green light to a Turkish attack on the Kurdish partners themselves, whose close ties to Kurdish militants in Turkey have long worried Ankara.

Trump ordered the withdrawal of US troops training Syrian Democratic Forces; Turkey then launched a violent campaign to drive Kurds away from the Turkish-Syrian border. With the SDF dispersed, IS seems ready to exploit the chaos. Reports that ISIS fighters have already escaped from Kurdish-run prisons have raised fears that extremism may arise from ashes in Syria.

The fact is that IS and Al Qaeda were about to return even before Trump withdrew and the Turkish invasion. Further conflict would only fuel extremism, which would once again destabilize the region and pose threats to Israel, Europe and even the United States. Unfortunately, a few years from now, Syria will return to what it was before the campaign against ISIS: suffering from chaos and conflict, as terrorism escalates.

Idlib chaos

The last piece of the organization in Bagour fell to coalition forces in March, and even ISIS leaders were busy reshaping their group in the countryside. Tens of thousands of fighters on the banks of the Euphrates died, but thousands more retreated to safe havens in the Syrian and Iraqi countryside and survived, including group leader Abu Bakr al-Baghdadi.

The extremist organization is preparing the battlefield militarily, politically and psychologically so that it can proceed with the offensive once the US-led coalition withdraws.

Al-Qaeda likewise adapts and gathers force in western Syria. The group thrived in the chaos of Idlib province, the last remaining stronghold of the opposition, which was attacked by pro-regime forces earlier this year. Idlib was once home to a mix of moderate, militant and extremist groups, and a bastion of militant militants controlled by al-Qaeda-linked HTS. Groups of al-Qaeda veterans separated from the body in 2017 to form a new group called 'Guardians of the Deen'.

The two organizations already had the opportunity to rebuild thanks to the American disengagement. Beginning in April after the fall of ISIS, the United States reduced its presence in Syria by half to 1,000 troops. The Pentagon said in August that "the reduction of US forces has reduced the support available to Syrian partner forces," and that the SDF can no longer "continue long-term operations against ISIS fighters."

Now, US support for Kurdish forces in eastern Syria, which includes specialized training in patrols, police, governance and intelligence gathering, is set to evaporate completely as US troops leave.

The rise of extremism

Turkey's invasion of Syria makes the creation of ISIS a reality. Ankara's ultimate goal is to exterminate the Kurdish entity in northern Syria, which is currently considered a bulwark against ISIS's control of that area. In the face of this existential threat, the YPG will surely use its close links with Kurdish militants inside Turkey to intensify the Kurdish insurgency there, and the YPG will thus shift its attention from ISIS to Turkey.

Indeed, the SDF appears unable to secure thousands of ISIS fighters in its custody, nor can it keep the tens of thousands of ISIS family members it was holding in the refugee camp under heavy security and with almost no international support. If chaos engulfs northeastern Syria in the coming months, ISIS could once again seize large swathes of territory for new expansion and send fighters across the insecure border to carry out terrorist attacks.

The Turkish offensive is also leading the SDF to strike a deal with Damascus, and the Assad regime aims to reaffirm its sovereignty over all of Syria's territory and prevent further foreign raids. As such, the Syrian government shares the SDF's vision of Turkey as a common enemy. In Aleppo in 2016, both sides worked together to eliminate the remaining Syrian opposition in the east of the city.

Indeed, the Kurds seem willing to relinquish control of the towns of Manbij and Kobani, and the sites of pivotal battles against IS for the sake of the regime and Russian forces.

Improve relationships

Such a deal would prompt the re-emergence of ISIS. Many Sunni Arabs in eastern Syria rebelled against Assad and considered the YPG rapists. Already.

IS will exploit domestic frustration to gain popular support no matter what the SDF chooses to do. Before the Turkish invasion, troops, encouraged by the United States, were working to improve their relations with the Arab population under their control. The anti-IS campaign has destroyed Arab populated areas in the region, and the local population was skeptical of the YPG, but the group was taking steps towards better and more inclusive management. In 2013, when Raqqa and Deir ez-Zor revolted against Damascus, IS took advantage of the resulting chaos by positioning itself as the only group capable of providing security and justice. This is likely to be a plan for the future.

The real goal

After eight years of war, culminating in Idlib, the Syrian army lacked manpower and motivation to fight another battle. Iran and Hezbollah are exhausted by the war and will not be ready to fight IS again unless the organization poses a real threat to key positions in western Syria and along the Lebanese border. Russia has little incentive to confront militants. Russian President Vladimir Putin wants to become a powerful and indispensable mediator in the Middle East. He is less interested in refugee flows, growing extremism and regional instability. By contrast, the more fragile Syria becomes, the stronger Putin will be in the region.

After decades of US involvement in Middle East conflicts that have caused countless casualties, drained economic resources, and made America an occupying power, Americans are right to beware of endless wars or engage in indefensible nation-building.

But the US-led coalition that toppled ISIL earlier this year was among the most efficient and cost-effective in history. The Trump administration failed to make its hard-won gains by withdrawing US troops and abandoning its Kurdish partners. All of this ensured the resurgence of an extreme threat to the region, Europe and the United States. Soon, the United States will have to calculate the dire consequences of this decision.

Brian Katz and Michael Carpenter are experts in international relations

The US-led coalition that toppled ISIL earlier this year was among the most efficient and cost-effective in history.