Gaziantep, Turkey (CNN) Sometimes when a complex mess suffers an added complication, it can in fact make matters simpler -- such is the case in northern Syria now.

For months, US support for the Syrian Democratic Forces (SDF), a predominantly Syrian Kurd force that also includes Arab tribes and other factions, has led them to take large swathes of territory from ISIS quite effectively and establish Kurdish control over the territory.

But there's always been one problem: that territory is along the Turkish border and Turkey considers the SDF's main component, a Syrian Kurdish militia called the People's Protection Units -- or YPG -- to be terrorists

For months the "what if" of Turkey's eventual reaction to these Kurdish successes has hung in the air, a potentially huge complication for US policy here, given that the US backs both sides. It's safe to say Ankara would oppose a de-facto Kurdish state springing up along its border -- and intervene.

Last week, things got a little clearer. Turkey did intervene, using the second largest army in the NATO alliance to back up Sunni Arab Syrian rebels, as they moved to take the Syrian border town of Jarablus from ISIS , almost without a fight.

But it did not stop there.

The same forces moved west along the border, expanding the valuable frontier territory now controlled by their Syrian rebel proxies. They also moved south, and ran, inevitably, into the US-backed Syrian Kurdish forces. The mainly Sunni Arab rebels consider this area theirs; and the Kurds view it as land they have fought hard for. With the Syrian Kurds being terrorists in Turkey's eyes, this is never likely to end well.

Caught in the middle

As we have seen before with the use of chemical weapons in Syria, Washington hoped they could draw a "red line" not to be crossed and all would be well. Vice President Joe Biden told the Kurds they would have to stay east of the Euphrates river, and withdraw from the Syrian town of Manbij -- which was retaken from ISIS with US assistance. The river provides a clear geographical boundary, running north to south, between areas the Kurds have controlled for a while, and fresh territory regained from ISIS.

The Pentagon said Sunday they thought that withdrawal east was happening. But, as of now, it is not. Instead, US Secretary of Defense Ashton Carter was left Monday to ask the Turkish not to fight the Syrian Kurds , and for the Syrian Kurds to pull back across the Euphrates. This, essentially, was like asking two sets of allies to disobey their instinctive impulses.

As of Tuesday, there was still no clear sign the Kurds were moving east. And on Wednesday Turkey's EU Minister Omer Celik denied that a ceasefire agreement had been reached between Turkish-backed rebels and Kurdish rebel groups following reports of clashes, state-run Anadolu reported.

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But this American balancing act between the two sides is unlikely to last long. The Syrian Kurds don't have the manpower to take on the full might of the Turkish military. They also won't want to irritate their American backers to the point where Washington is forced to abandon them (and their priceless airpower) in an attempt to save its fragile relationship with Turkey.

There is a basic logic to this. US policy in Syria would be at a total loss without Turkish support. They need Ankara's blessing for resupply lines to their special forces and allied rebels all over Syria. They need it for airbases to launch anti-ISIS strikes from. They can't genuinely believe any long-term solution for Syria is possible without Turkey, its NATO ally, being on board.

Friend or foe?

How far will Turkey go?

The Euphrates seems a sensible, obvious demarcation line. It would allow the Syrian Kurds to save face, and keep a hold of the strategically important city of Kobani and a large swathe of northern parts of Syria. It would allow the Turks to create a buffer zone in northern Syria, focus on kicking ISIS out of other key towns like al-Bab, and let the Syrian Sunni rebels they back run a large chunk of the border region.

Turkey has the second-largest military in NATO

This is something that has oddly been a key tenet of US policy for a long time that it has resolutely failed to achieve in Syria: to establish a functional, Sunni Arab rebel force in the north. The US hoped they could represent the interests of the millions of disenfranchised Syrian Sunnis who've been kicked around and bombed since the war began. Some were so desperate and angry about their situation they initially viewed ISIS as liberators

This Sunni Arab bulwark is what the US has long sought too, to take on ISIS. You cannot send Syrian Kurdish forces to clear ISIS from Sunni Arab towns and expect a happy outcome. For future stability, the Syrian rebels Turkey is backing offer a better alternative to fight ISIS than the US's current anti-ISIS ground force -- the Syrian Kurds.

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So here we are, with months of uncertainty swept away. Turkey wants the Kurds to move back, so does Washington. America wants the Turkish to leave the Kurds alone and fight ISIS. ISIS are on the back-foot and have -- regardless of the white noise of Turk versus Kurd -- just lost control of the border town they badly needed to resupply themselves.

It's now clear who wants what, and relatively clear who is doing what, and who is not. This could herald an opportunity, a golden moment in which Turkey helps Sunni Arabs back on their feet again in Syria, while Syrian-Kurdish territorial ambition gets a reality check.

Except, sadly, this is Syria, where any sense of clarity is quickly overcome by further animosity and chaos -- where anything decent is eviscerated.