The US and Turkey's long-term plans for northern Syria
With both powers clearly gearing up to stay in Syria for the long haul, their presence is bound to become ever more contentious as the Syrian war transitions into a new phase.
US Secretary of State Rex Tillerson visited Ankara this month as the two NATO allies' divergent agendas in Syria threatened to lead to direct confrontation on the battlefield.
Since late 2014, the US military has worked with the Syrian Kurdish People's Protection Units (YPG) - and the larger Arab-Kurdish Syrian Democratic Forces (SDF) coalition, of which the YPG forms the backbone - to destroy the Islamic State group.
Turkey views the YPG in the same light as its eternal foe, the Kurdistan Workers' Party (PKK), and is therefore concerned over US support for the YPG, leading to a serious strain in relations between Ankara and Washington.
Tillerson sought to salvage the collapsing relationship during his recent visit. A particularly dangerous flashpoint exists between the Americans and Turks in Syria in the city of Manbij on the west bank of the Euphrates River.
Captured from IS by the SDF in the summer of 2016, Manbij remains under the group's control. Turkey has threatened to attack it on numerous occasions, claiming that the YPG remains there in direct contravention of a US promise.
In spite of the continued presence of US Army Rangers in Manbij, deployed last March to prevent clashes breaking out between the YPG and Turkey's allied Syrian fighters under the banner of the Free Syrian Army (FSA), Ankara has reiterated its vow to attack - even going so far to warn that harm could befall those US troops if they get in the way of a Turkish attack.
|If implemented then the most serious, and dangerous, bone of contention between the two NATO powers will be resolved
Following his meeting with the Turkish leadership, Tillerson said such tensions were being consigned to the past. "We are not going to act alone any longer, not US doing one thing, Turkey doing another," he said.
"We will work together... we have good mechanisms on how we can achieve this."
One Turkish suggestion for resolving the Manbij impasse is for the joint administration of the Syrian city by US and Turkish troops, which would allow Ankara to verify the absence of the YPG.
If implemented, then the most serious, and dangerous, bone of contention between the two NATO powers will be resolved - possibly paving the way for the kind of cooperation Tillerson advocates.
|Kurds currently control Afrin, Kobane and Jazira provinces
The Syrian regime opposes the presence of both these armies, since they are there without its authorisation - unlike the Russian military and Iranian proxy militias upon which President Bashar al-Assad depends.
Unless it deploys forces on Afrin's border with Turkey - which the Afrin YPG welcomes as a necessary counter-measure against the ongoing Turkish operation - Damascus has no real presence on its northern borders, something it doubtlessly seeks to rectify having recaptured most of the country in recent years.
Turkey justified its military's presence in northwestern Syria - as well as its operation against Afrin - as defensive under Article 51 of the UN Charter. It also maintains that its FSA allies are a Syrian national army, which in turn legitimises Ankara's continued presence in the 60-mile swath of northwestern border territory captured in the Euphrates Shield operation [August 2016 - March 2017], since they are allied with this reportedly 22,000-strong "national army".
Read more: Turkey will lay siege to Syria's Afrin in coming days, warns Erdogan
Washington has approximately 2,000 troops in Kurdish-held areas of north-east Syria - territory which extends across two-thirds of Syria's northern border, from Iraq to the east bank of the Euphrates.
Opposition to this presence by Damascus, Moscow and Tehran has increased incrementally in recent months as the IS threat decreased. Damascus doesn't recognise the federal system of governance established by the Kurds in the territory they hold, and some opposition figures in exile likewise see growing efforts for Kurdish autonomy as a dangerous path leading to the potential break-up of Syria as a country.
Kurdish territories combined with areas captured from IS in Deir az-Zour and Raqqa include some of Syria's best agricultural land, the majority of its oil reserves and Tabqa Dam - the Syrian equivalent of Egypt's majestic Aswan Dam.
|Washington says its continued presence is necessary for post-IS stabilisation
While the US denies it has any nation-building goals in Syria it seeks to build-up the SDF/YPG and has demonstrated its ability to swiftly defend them - most recently by heavily bombing and driving back a combined force of pro-regime militia fighters and Russian "mercenaries" who attacked an SDF base in Deir az-Zour on 7 February.
As The Atlantic pointed out in January, the US troop presence in northern Syria is being "joined by a growing army of diplomats and aid workers... overseeing hundreds of millions of dollars worth of reconstrution and security projects".
Turkey has essentially been doing the same thing in the north-west for almost a year now, by building local security forces and sending "civilian officials" to oversee reconstruction.
Washington says its continued presence is necessary for post-IS stabilisation and has spoken of remaining in Syria until the conflict is brought to an end, which means they could end up remaining until Syrian President Bashar al-Assad either relinquishes power as part of a settlement or is somehow removed.
Turkey has highlighted the humanitarian end of its presence in the northwest - the so-called triangle of territory which edges are marked by the cities of Jarablus and Al-Rai on its border and Al-Bab 30 kilometres south of that border - which has consisted of resettling displaced Syrians there.
Ankara also says it ultimately aims to return the 3.5 million Syrian refugees currently in Turkey to these areas. With its relative stability and resources, northern Syria under Kurdish control also has the clear potential to accommodate a large number of Syrian Arabs from elsewhere in the war-torn country.
These separate American and Turkish commitments and investments in northern Syria to date, coupled with their long-term plans for the future of these territories, clearly signal that they have no intention of leaving of their own volition anytime soon.
Paul Iddon is a freelance journalist based in Erbil, Iraqi Kurdistan, who writes about Middle East affairs.