Last Thursday an attack struck Turkish positions near the Islamic State-held city of al-Bab in northern Syria, killing three soldiers and wounding 10. Centered between Turkish and rebel forces in the north, the Kurds in the east and west, and the Syrian regime forces in the south, al-Bab is the point of convergence for the three competing interests in northern Syria. Conflicting reports on the perpetrators of Thursday’s attack are indicative of the complex situation with a variety of hostile actors concentrated only miles apart, with inevitable clashes among them looking closer than ever. Whether the perpetrator of Thursday’s attack will prove to be the Islamic State (ISIS) or Russian-backed Syrian regime forces, Turkey is on a collision course that it may not be prepared for.

Immediately following the attack, Turkish sources blamed the Syrian Arab Air Force (SyAAF) and issued a media ban to block inquiries into the situation. Falling on the first anniversary of Turkey’s downing of a Russian jet – an incident which caused a nine month diplomatic spat between Ankara and Moscow – the attack prompted speculation of retaliation by Russia – or by the Russian-backed SyAAF. Indeed, back and forth phone calls between the Turkish and Russian Presidents in the 48 hours following the incident strengthened such suspicions. In the meantime, however, other reports emerged indicating the attack could have been a suicide bombing by the Islamic State.

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Whoever the perpetrator, the assault should serve as a wake-up call for Turkey as it advances towards al-Bab, challenging each of its many adversaries on the ground head on. With each side committed to its own gameplan, a military confrontation between the Turkish-led Euphrates Shield and pro-regime forces appear inescapable. More generally, Thursday’s attack should be an acute reminder that tensions between Ankara and Moscow are a mere spark away from reigniting the flames of only a year ago.

Since launching its intervention in Syria in August, Turkish forces partnering with Free Syrian Army (FSA) branded units have advanced south into ISIS-held territory, increasingly close to regime forces besieging Aleppo in the southwest. While the stated goal of the operation was to push the Islamic State away from the Turkish border, Ankara’s other – and all the more important – aim has been to block Kurdish territorial expansion through the Aleppo Governorate. The successful capture of ISIS-held Manbij by the Kurdish-dominated Syrian Democratic Force (SDF) in early August, combined with advances by the Kurds from the Afrin Canton in the west, was cause for alarm for Turkish leaders. Ankara has been engaged in a bloody war with the Kurdish-separatist Kurdistan Workers’ Party (PKK) at home for four decades, and does not differentiate between the group and its Syrian affiliate, the Peoples’ Protection Unity (YPG). Since 2014, the YPG’s fight against ISIS has allowed the Kurds to extend their territorial control in northern Syria. With the liberation of Manbij, they came close to controlling a nearly 400-miles of Turkey’s 500-mile border with Syria.

Further compounding the complexity is the US-led Coalition’s support for YPG advances in Syria, reluctantly seeing the Kurdish group and its SDF allies as the only effective local partner to destroy ISIS. This pragmatic partnership has been a major source of tension between Ankara and Washington for over a year, and the reason why Turkey launched operation Euphrates Shield without coordination with the anti-ISIS Coalition. While the US and Turkey did temporarily come to an agreement for the US to support Euphrates Shield’s southward advance, poor coordination and a lack of trust between the two NATO allies has resulted in the US pulling its air assets and special forces from participating in the effort to seize al-Bab.

Without U.S. air cover, Turkish forces are particularly vulnerable to SyAAF or Russian airstrikes if al-Bab is indeed a red line for the regime as reports suggest. The Turkish Air Force (TAF) is desperately low on pilots in the wake of July’s coup attempt and the subsequent purges of the military’s officer corps (reports indicate more than 350 pilots, including many of the force’s most experienced, were dismissed). Even if fully staffed, the TAF would struggle to penetrate Syrian air space because of the formidable Russian air defense bubble over western Syria – which extends to al-Bab and deep into Turkey.

Yet, Turkey appears undeterred, with Turkish President Tayyip Erdogan declaring that Turkey’s intervention is to “end the rule of the cruel Assad.” Moreover, the FSA branded factions fighting alongside Turkish troops are determined to advance through al-Bab and towards regime lines to help relieve their besieged comrades in Aleppo. Once al-Bab falls, whether to Turkish and FSA forces or pro-regime elements, the buffer that now separates Euphrates Shield and the regime’s eastern flank around Aleppo will be too small to prevent clashes. Such a scenario risks pulling the United States further into the Syrian theater – not to strengthen the fight against ISIS, but to defend an incorrigible NATO ally.

Patrick Megahan is a research analyst on military affairs at Foundation for Defense of Democracies, where Merve Tahiroglu is a research associate focused on Turkey. Follow them on Twitter: @PatMegahan and @MerveTahiroglu

This article originally appeared in The Long War Journal.