The emergence of the Islamic State militant group as the most powerful opposition force in the Syrian civil war had the salutary effect of uniting the disparate forces in that conflict in a single purpose: defeating and destroying the still more bloody extremist group. It is ironic then that as the prospect of that defeat now appears within grasp, there is the equally likely prospect of a wider and more dangerous fight over the future of Syria and the region, a struggle that threatens to pit the West, particularly the United States and its regional allies, against Iran. And, as a side show, there is the chance of a clash between the U.S. and Russian militaries as well.

IS was a coalition of fundamentalist groups that had emerged from the disintegration of Iraq and the civil war in Syria. In 2014, it made substantial gains on the ground, and at its peak controlled as much as 45 percent of Iraq, making real their dream — and others’ nightmare — of a caliphate. Those dreams reached their feverish peak nearly three years ago, on July 4, 2014, when Abu Bakr al-Baghdadi, head of IS, stood at the pulpit of the 12th century Great Mosque of al-Nuri in Mosul and declared the creation of a caliphate, or a Muslim state, in land the group controlled in Syria and Iraq. Under the black IS flag, he declared a jihad to restore Islamic supremacy over the world.

Last week, as Iraqi and allied forces looked set to retake Mosul and crush the dream of an IS caliphate, IS destroyed the al-Nuri mosque and its famous leaning minaret in a final act of defiance and pique. This act of cultural terrorism — only the most recent among many — was rightly acknowledged by Iraqi Prime Minister Haider al-Abadi as a “formal declaration of their defeat.” Jan Kurbis, the United Nations special envoy to Iraq, added that “the destruction … shows their desperation and signals their end.”

The victory in Mosul was the product of an eight-month offensive, and while important will leave pockets of remote IS-held territory in Iraq and Syria. Perhaps most dangerous is the threat of IS-inspired lone-wolf terrorists acting in the West, as has occurred in recent months. Ironically, while those attacks bring the threat of IS home to Western audiences, they are in fact a reflection of the group’s weakness, not its strength.

But as IS retreats, the odds of a different conflict are growing. U.S. and Russian militaries, which back different groups in the Syrian civil war, are coming ever closer as their proxies try to fill the vacuum created by the IS retreat. Those proxies faced off four times in June, with the U.S. Air Force shooting down a Syrian Air Force plane, along with two drones, which resulted in the suspension of U.S.-Russian cooperation to avoid such a conflict. In the aftermath of allegations of Russian meddling in the 2016 U.S. presidential election and the imposition of new U.S. sanctions against Moscow as a result, U.S.-Russia relations have continued to harden and there are growing fears that either side could provoke the other to make a larger political point.

Troubling as that may be, there is another, even more pressing concern: the prospect of a wider war with Iran. Iran, more than any other country, is positioned to exploit the IS retreat. Iranian forces and proxies are the most powerful throughout the region and Tehran has had no hesitation about exploiting opportunities to extend its influence. There is a fear that the defeat of IS will allow Iran to establish a corridor to project its power from the Persian Gulf to the Mediterranean Sea.

While U.S. President Donald Trump argued as candidate that U.S. involvement in Middle East wars was a mistake and a huge drain on U.S. coffers, he was equally insistent in identifying Iran as an enemy of the West and called on other nations to unite to contain Tehran. The key theme of his May visit to Saudi Arabia was recognition of the Iranian threat, a message that the Saudi government in Riyadh was delighted to hear and has taken to heart.

Sunni leaders in Saudi Arabia and allied states are eager to take the fight to Iran to counter its growing power and to forestall any challenge to their dominance of the region. It is no coincidence that immediately after that visit, Saudi Arabia and some of its allies attempted to isolate and bully Qatar and some see Saudi influence behind two terrorist attacks in Tehran that followed the trip (even though IS claimed responsibility). It is illustrative of the complexities of the Middle East that defeat of a deadly foe threatens a wider, even more destabilizing war.

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