Ankara's military build-up on Syria's border heralds tough choices for Washington

Turkey has already shown a willingness to match words with actions in Syria.

It launched Operation Euphrates Shield in 2016 to capture Jarablus and al-Bab from IS as part of an operation aimed at preventing a "terror corridor". More recently, Operation Olive Branch in January captured Afrin from Kurdish forces.

However, any new military operation will have deeper, long-term ramifications.

Turkish President Recep Tayyip Erdogan's vow to launch the incursions "within days", was met with a strong rebuttal from Washington.

Commander Sean Robertson, a Pentagon spokesperson, deemed any operation to cross the border as "unacceptable". "Unilateral military action into northeast Syria by any party, particularly as US personnel may be present or in the vicinity, is of grave concern," he stated.

Yet, while the SDF remains the most effective US local partner in Syria, who have been crucial to defeating IS, the US has been stuck between the tactical alliance with the SDF and strategic ties with its NATO partner and traditional regional ally.

The US has to start making difficult long-term choices between their two allies

Evidently, the Ankara-Washington relationship has significantly cooled in recent years, and not just over Syria. The eventual release of US Pastor Andrew Brunson, may have helped to improve ties on the surface, but the months of harsh rhetoric, threats and trade sanctions cannot be easily papered over.

Erdogan repeated a long-time threat that Turkish forces will enter the mixed town of Manbij, west of the Euphrates, if the US does not remove YPG forces. Joint patrols between Turkish and US forces, aimed at soothing Turkish concerns, were never going to appease a Turkey with eyes on Kurdish positions far beyond Manbij.  

Erdogan accused the US of protecting terrorists as opposed to protecting Turkey from possible action. He also held a phone call with US President Donald Trump in recent days, in which they agreed on more effective coordination in Syria, but any US concession is likely to fall well below Turkish demands.

The strong warnings of US officials over any new Turkish incursion, joint border patrols between US and SDF forces, and lately the building of new US observation posts on the border, all suggest that the US is not about to forgo their Kurdish partners.

However, US policy on Syria in particular and the SDF in general has been murky and inconsistent. It has been largely fashioned around the fight against IS. Now, with SDF forces close to capturing the last major IS stronghold of Hajin, what will the US do next?

The US has tried to sit on the fence between the local allies in SDF and regional allies in Ankara but this situation is untenable.

As demonstrated by the lack of intervention in Afrin, US support is hardly guaranteed

If the US deems the fight against IS complete, will it abandon the SDF and place them at the mercy of Ankara?

Not only will this bring a new bloodbath and add to the complexities in Syria, it will set back the relative stability in Syria and potentially bring Syrian opposition forces as well as Damascus, Tehran and Moscow into a new deadly mix.

Meanwhile, such new instability leaves the door ajar for IS to regroup, gather strength and scupper US goals in the region.

The US has to start making difficult long-term choices between its two allies. It has to signal its long-term intentions in Syria as well as their partnership with the Syrian Kurds. In time, it will need to take bold measures to appease both sides, as impossible as that seems at present.

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These measures include a long-term roadmap reaching beyond the fight against IS. Measures such as continuing to support the SDF in its present shape, while putting bounties on the heads of senior PKK leaders, will not soothe Turkish concerns.

Once the dust settles on the battle, the status of the Kurdish region is still in need of a political solution enshrined by a new constitution in Syria. The Syrian Kurds remain divided between those favourable to the more dominant Democratic Union Party (PYD) and those groups leaning closer to the leadership of the Kurdistan Region.

Then comes the difficult job of rebranding the image of the YPG and the region in general, away from what Turkey feels is a backyard for the PKK. A sense of wider political representation, including those factions backed by the Kurdistan Democratic Party (KDP) which Ankara sees more favourably, bolstered by new local elections would help dilute the dominance of the PYD.

Of course, none of this might be enough to placate Turkey, which is where future US redlines and strategy come in.

The US needs a continued presence in Syria, not only to ensure IS remains on the back foot, but also to guarantee a role in the future makeup of Syria, and to keep Iranian regional aspirations in check.

As for the Syrian Kurds, without the long-term guarantees of the US or a political roadmap that safeguards their hard-fought gains, the relative reassurance that US presence provides is insufficient.

Firstly, as demonstrated by their lack of intervention in Afrin, US support is hardly guaranteed.

Not only does the PYD need to make concessions to the Kurdish parties and forces seen as more acceptable to Turkey, and attempt to distance itself from the PKK, but it must keep the political and strategic channels with Damascus and Moscow firmly open.

Any political deal with Bashar al-Assad may not be easy to stomach, but the Kurds have little choice. They refused to hand over Afrin to Damascus as a condition of support, and ended-up losing the city altogether to Turkish backed rebels.

Meanwhile, Russia has enjoyed relatively warm ties with the Kurds and has tried to ensure the Kurdish groups have a seat at the peace table, but a long-term pact with the Kurds must serve Moscow's regional goals.

This includes discrediting and diluting the US influence in Syria, seeing the Kurds lean away from Washington, and ultimately a scenario that leads to as much control for Assad over all parts of Syria.

Both Washington and the PYD have difficult choices to make, but the latter has by far the most to lose if it does not play its cards carefully.

The US is not about to forgo their Kurdish partners

Turkey has long-term dilemmas of its own. The ongoing threats against the Kurds serve to pressure the US, but also to placate a nationalist voter base ahead of local elections next year. It also wants to safeguard its long-term foothold in Syria where there is little to suggest that Ankara will readily relinquish strategic areas under its control either now or in the future.

Either way, Turkey is unlikely to ever thwart the threat of PKK or the YPG for that matter with mere military action. If military action was an ultimate answer, the PKK, whose military capability is far dwarfed by that of Turkey, would have been long eradicated.

On the contrary, Turkey can ill-afford a costly long-term military adventure in Syria, and must also make difficult compromises to safeguard its own regional interests.