Why are Turkey and the US falling out over Syria?

The US Pentagon has announced plans to officially begin sending arms shipments to an allied force on the ground in northern Syria, known as the Syrian Democratic Forces (SDF). According to White House Press Secretary Sean Spicer, the weapons will specifically be delivered to "Kurdish elements" of the SDF ahead of a push to liberate the city of Raqqa from Daesh.

But the Trump administration's new policy is likely to anger Turkey, a key NATO ally and Washington's long-term partner in ensuring security in the region.

Here are four reasons why:

1. The US is ignoring the YPG's terrorist links

The Pentagon's announcement refers to arming the YPG, which comprises the largest bulk of SDF members. Turkey considers the YPG to be a terrorist organisation due to its links to the PKK.

The PKK has waged an armed insurgency against the Turkish state since 1984, which has claimed some 40,000 lives.

Although the US also recognises the PKK as a terrorist organisation, as do many other countries and international bodies including the EU, Washington insists that the YPG is a separate organisation that has no ties to the PKK. This position is even at the expense of testimony provided by former Secretary of Defense Ash Carter, who admitted the link between the two.

2. Arming the YPG is no different to arming the PKK

Turkey has long expressed fears of such weapons being transferred from the YPG in Syria to the PKK and being used against the Turkish state.

Cross-border fire coming from YPG-controlled territories in Syria has already hit neighbouring Turkish provinces. A Turkish intelligence report also revealed that a suicide bomber who killed 37 people and injured 125 others in the Turkish capital Ankara in March last year had crossed into Syria in 2013 for training by YPG.

It is not clear as yet what type of equipment the US will be providing the SDF. Analyst Kevin McAleese speculates that bulldozers, engineering equipment, rifles, ammunition and communications gear are likely to be included in the shipments.

Earlier reports suggest the US has already provided support for the YPG in the form of armoured vehicles, weapons and ammunition. But until now, the US had denied Turkish accusations of arming the YPG.

3. Strengthening the YPG threatens Turkey border

The YPG currently controls an uninterrupted 400-kilometre (250-mile) stretch of Turkey's border with Syria, and was on the verge of taking complete control before Turkey launched Operation Euphrates Shield in August 2016.

The operation cleared a strip of land between the YPG-controlled enclaves of Afrin and Manbij from Daesh and handed control of it to the Turkish-backed opposition Free Syrian Army (FSA). This secured a land corridor that would continue to link Turkey to the rest of the Middle East.

But the US arms shipment to the YPG could embolden the group in its bid to establish its own corridor that will connect them to their estranged enclave of Afrin. Such an outcome could destabilise Turkey's entire southern border as it will provide a safe haven for the PKK and threaten Turkey's trade routes in the region.

4. The YPG is not the right choice for the fight against Daesh in Raqqa

US insistence on the SDF's involvement in the operation to liberate Raqqa from Daesh has sidelined Turkey. Although US Secretary of Defense James Mattis said after Tuesday's announcement that Washington still intends to work with Turkey in taking Raqqa, Turkey has long refused to participate in the operation if the YPG is involved.

Turkey instead wants the US to cooperate with the FSA in fighting Daesh in Syria, especially considering that Raqqa is an Arab city and its residents may not take kindly to being controlled by the YPG.

Raqqa is also an oil-rich province, which could potentially become a source of income for the YPG if they take control.