Turkey's operations in Syria are informed by the twin domestic threats of Kurdish separatist and Islamist terrorism. While the West supports groups linked to the Kurdish PKK terror group, it risks a clash between the proxies of two NATO powers. Meanwhile, al-Qaeda-linked groups can grow in power.

While all international efforts to find peace in Syria have ended in stalemate, the most pressing question for international powers is how to defeat ISIS in Raqqa. Two powers with the greatest interest in defeating the group, Turkey and the United States, have found themselves backing rival forces on the ground. The consequence may well be new dangers to the West.

Washington is sponsoring the Syrian Democratic Forces (SDF), a coalition dominated by the Syrian Kurdish People's Defence Forces (YPG), which is alleged to have ties to the Turkish separatist terrorist group the PKK. Meanwhile, in August 2016 Turkey launched the 'Euphrates Shield' coalition of Syrian rebel groups including Jaish al-Islam and parts of the Salafi coalition Ahrar al-Sham. This coalition has a twin goal: to fight ISIS, and to fight Kurdish forces connected to the PKK. Both coalitions are advancing on Raqqa.

In early March, Turkish President Recep Tayyip Erdogan announced that the Euphrates Shield forces would move on the SDF-held city of Manbij, as part of its advance on Raqqa. Washington has deployed troops to Manbij – as advisors and for intelligence gathering – who will be caught up if conflict breaks out between the SDF and Euphrates Shield.

In the narrow context of the Syrian conflict, the choice of proxies that the US and Turkey have made is logical. Both are committed to fighting ISIS. Turkey has long been opposed to Kurdish nationalists with any link to the PKK, responsible for tens of thousands of deaths in Turkey, and designated a terrorist organisation by NATO. Strategically, Turkey has the twin incentive of driving a wedge between SDF-controlled areas on its border, and proving its value in the fight against ISIS. Washington, meanwhile, has been discouraged by the paucity of 'moderate' and effective forces within the rebellion, and regards an alliance with the secular nationalist ideology of the YPG attractive.

As with most disputes between rebel factions, the conflict between the SDF and the Euphrates Shield coalition is a gift to the regime. After Erdogan's announcement of an advance on Manbij, SDF forces withdrew from the west of the town, allowing regime forces to fill the gap as a buffer. A confrontation between the two groups could leave the US in a similar position, with its troops effectively allied with regime forces to keep the peace between rebel factions.

Yet the threat posed by this dispute goes beyond a possible clash involving US troops. With the US and Turkey supporting different factions in the march on ISIS' core territories, two coalitions can seize the vacant space in northern Syria: Tahrir al-Sham coalition, dominated by groups that share ISIS' ideology and pose a direct threat to the West, including al-Qaeda's former affiliate; and the majority of Ahrar al-Sham, a Salafi-jihadi grouping whose founders had ties to al-Qaeda.

The assault on Raqqa should be an opportunity for the US and Turkey – NATO allies too frequently at loggerheads over Syria – to come together to fight a shared enemy. Instead, they may find themselves in a situation where their proxies are in active conflict. Their opposing interests are a gift to ISIS, though the group will sooner or later be defeated. More troubling for the long-term security of Turkey and the West is the way the dispute is empowering our other enemies.