The US has not been spoilt for choice when choosing what, if any, side to support in Syria. In part this is because opposition rebel groups have struggled to put their differences aside and work together to overthrow Bashar al-Assad’s regime; but it also due to misguided and miscalculated US engagement in the conflict, and the region. And the west’s shift of focus to Islamic State stifled the campaign to overthrow Assad.

In this bloody and uncertain proxy war, the decision to work closely with the Syrian Kurdish group the Peoples’ Democratic party and its armed wing, the Peoples’ Protection Units (YPG), was an easy one. The secular, western-friendly group has been fighting jihadi forces long before Isis came on the scene; and by playing a largely neutral role in the conflict – refusing both to fight the Assad regime, and to join opposition forces that refused to accommodate Kurdish demands – took the opportunity to establish its own autonomous region in north-east Syria.

The YPG has been instrumental in retaking territory from Isis. And it will be at the forefront of the coming battle to retake Raqqa as part of the Syrian Democratic Force, the umbrella militia that is ethnically dominated by Arabs but is spearheaded by the YPG and its Kurdish fighters. Washington’s decision on Tuesday to directly arm the YPG and equip it with heavy weaponry is a game-changer for US relations with Syria’s Kurds – and, potentially, for the way western governments engage with armed groups, to reflect the emerging regional order.

Supplying the YPG in this way comes with responsibilities. Turkey fears the ascendancy of the YPG, which is the sister-group of the Kurdistan Workers’ party (PKK), an organisation that has fought the Turkish state for 40 years. What worries Ankara is the emergence of an autonomous Kurdish region along its borders acting as a launchpad for attacks on Turkish soil, and this concern cannot be ignored. Isolating the second-largest military force within Nato – a traditionally indispensable component of the western-led liberal international order (an order that, granted, might be debatable today) and a regional power that constrains the hegemonic ambitions of Iran and Russia – could bring more costs than benefits for both the region and the international community.

Turkey can disrupt Syrian Kurdish ambitions and the YPG-US partnership. Its threshold for engaging the PKK and the YPG is much lower than its threshold for directly engaging the Assad regime, which it has sought to undermine by backing Syria’s rebels. The PKK is a group that Ankara understands, has studied and has a proven capacity for engaging militarily. It is no stranger to conducting full-scale military incursions in pursuit of the PKK and has historically done so in northern Iraq, where the PKK has established affiliates, and where the structural conditions are much more conducive to a military incursion.

‘The YPG may be too important for the US to drop.’ Members of the YPG – the Kurdish Peoples’ Protection Units – during a lull in the fighting near the Isis stronghold of Raqqa. Photograph: Delil Souleiman/AFP/Getty Images

There is also an intra-Kurdish dimension to the issue. While the PKK has waged its insurgency for a combination of political, cultural and territorial rights for Turkey’s Kurds, it is also widely disparaged among the different Kurdish communities of the region. Hailed as heroes in 2014-15, when the PKK took on Isis in Sinjar, in Iraqi Kurdistan, the group is now seen to be undermining the Kurdistan Regional Government (KRG), which has asked the PKK to leave the area. In undermining the KRG, the PKK has also undermined US and coalition efforts to stabilise northern Iraq in preparation for the liberation of Mosul and its aftermath.

The PKK’s affiliates have exploited the security vacuum that emerged in 2014 to undermine the politics and security of northern Iraq. They have militarily confronted a sizable Syrian Kurdish force in Iraqi Kurdistan known as the Rojava (Syrian Kurdistan) Peshmerga, which is engaged in the Mosul operation and has been prevented by the YPG from returning to Syria to help combat Isis.

The US must use its military partnership with the YPG to make it a more inclusive, pluralistic actor

Since the US-led campaign and the recapturing of towns and cities in northern Iraq, the PKK has also formed alliances with Iranian-backed Shia militia proxies in northern Iraq, as both Iran and the Baghdad government seek to destabilise Iraqi Kurdistan. Iran and its proxies have taken advantage of their ties to the PKK to use the area as a transit point for their entry into Syria, where they are fighting alongside the Assad regime.

In doing so, the PKK has shown that it is happy to weaken its Iraqi Kurdish counterparts and happy to undermine US interests in Iraq to further its own expansionist ambitions. Both the US and its Syrian Kurdish allies will take two steps back in both Syria and Iraq if overlapping Turkish-Kurdish and intra-Kurdish dynamics are not factored into the anti-Isis campaign in Syria and Iraq. The YPG may be too important for the US to drop, but that does not mean Washington and its European allies cannot invest in the politics of northern Syria and northern Iraq. The US must use its military partnership with the YPG to make it a more inclusive, pluralistic actor that works with other Syrian Kurdish actors, rather than one that engages in authoritarian governance and further consolidates its hold on power.

Underinvestment in the politics, and lack of appreciation for the transnational nature of the problem has emboldened the west’s rivals, such as Iran and Russia. Yet aiming for an inclusive political system in Syrian Kurdistan will go a long way towards stabilising northern Syria after Raqqa is retaken: towards building on the liberation of Mosul, and reassuring powerful regional actors like Turkey.