Beirut (CNN) The dusty roads of Azaz, just off the Turkish-Syrian border, used to be the sort of place you whizzed through on the way to rebel-held Aleppo. Now it is the scene of seismic changes to the war in Syria , which could decide the most complex and unresolved question of the conflict: who will fight for Syria's Sunni Arabs?

Why this region matters: A rock and a hard place

It will help if you look at a map: Essentially this fight is for the slim tranche of territory these relatively moderate Sunni rebels used to consider their stronghold and that used to be the vital supply route into Aleppo.

In the past 10 days, all that has changed. The Assad regime has moved fiercely into the south of the area, cutting off the supply route to Aleppo. Then, in the past few days, the Syrian Kurds have moved east out of Afrin, the enclave north of Aleppo in which they had stayed peacefully for years, and taken territory quickly from moderate Syrian rebels.

On Monday night a leader with the Syrian moderates told CNN that the Syrian Kurds had taken Tal Rifaat, a vital town to the south of Azaz. This means they only have to advance mere kilometers east before they reach the front lines with ISIS that the Syrian moderate rebels have fought to defend for months. As of Tuesday afternoon, reports were emerging the Syrian Kurds had struck a deal to enter the key town on that front line -- Marea -- without a fight.

This leaves the U.S. as an awkward spectator to a fight between one group it has been supporting with limited success -- the Syrian moderate (Sunni Arabs) rebels -- and another group which has thrived with its assistance -- the Syrian Kurds. The U.S. will do what it can to pressure the Kurds to turn back, but it may be too late.

One of the Syrian moderate rebels told CNN he believed the Syrian Kurds were getting arms from the Russian and regime forces to their south. The rebel, who has strong ties to the U.S., told CNN: "We have strong evidence that (they) are coordinating with the Russians, who have been providing a lot of support. We have already submitted this evidence to the Americans."

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He echoed what many observing the Kurdish advances have feared: that they are part of a land grab by the Kurds to bolster their ambitions for a state of their own. This idea terrifies NATO member Turkey, which sees the Kurds as the major threat to its state. The Kurdish move east brings them closer to their dream of uniting Afrin with the town of Kobani and creating a contiguous Kurdish state along Turkey's long southern border.

"We will take part in the battle against them because they are trying to change the demographic of the land," the Syrian Sunni rebel told CNN. "It has been Arab land for thousands of years; they want to change it with Kurdish leadership. But they are dreaming. This will never happen."

Yet such sentiments may already be too late: The advances are swift and the Syrian Kurds have historically been successful at holding territory. For their part, the supporters of the Syrian Kurds claim they are cleansing the area of radicals among the rebel ranks. Things are moving very fast -- and the changes may alter the course of U.S. policy in the war.

But why is this tiny patch of land, and the ramshackle moderate rebels who hold it, so important? It's because they are -- in the north of the country, at least -- all that remains of the moderate Sunni Arab forces the U.S. has long courted to attack ISIS and, possibly later, confront the Syrian regime.

Their defeat could put an end to the international coalition's goal of finding a moderate Sunni force as a partner in the fight against ISIS and to represent the millions of Syrian Sunnis who began the rebellion against Assad. Granted, there are moderate forces in the south of Syria who have been getting Western aid, but they are mostly not fighting ISIS. And the pockets of moderates around the country that remain have been hit hard and are tricky for outside aid to reach.

It has been in the north that the bulk of the US effort has focused. For this reason, the advances around Azaz have had greater impact. It's been the failure of moderate Sunni Arabs in Syria to find a cohesive political and military voice that has been behind the rise of ISIS.

This is how we got here: Failure of moderates

Back in 2012, when the revolution became a civil war, most of the fighting against al-Assad was done by moderate Sunni Syrians: schoolteachers or plumbers who had taken up arms and sought a better life. They were bombed relentlessly by the regime, yet persevered. In many ways, they were this long war's first and indisputable heroes: protecting themselves and infected by the dizzy and unattainable ambitions of the Arab Spring.

Photos: Syrian refugees flee embattled city of Aleppo Syrian refugees await their fate near the Turkish border gate as they flee the northern embattled city of Aleppo on February 6, 2016. Hide Caption 1 of 17 Photos: Syrian refugees flee embattled city of Aleppo Refugees push each other as they wait for tents near the Turkish border on February 6. Hide Caption 2 of 17 Photos: Syrian refugees flee embattled city of Aleppo A refugee girl reacts near the Turkish border gate on February 6. Hide Caption 3 of 17 Photos: Syrian refugees flee embattled city of Aleppo Refugees jostle one another for tents near the Turkish border on February 6. Hide Caption 4 of 17 Photos: Syrian refugees flee embattled city of Aleppo Refugee children arrive at the Turkish border gate on February 6. Hide Caption 5 of 17 Photos: Syrian refugees flee embattled city of Aleppo Syrian refugees are pictured in a camp as they flee the city of Aleppo on February 6. Hide Caption 6 of 17 Photos: Syrian refugees flee embattled city of Aleppo A refugee carries a heavy bag of items near the Turkish border gate on February 6. Hide Caption 7 of 17 Photos: Syrian refugees flee embattled city of Aleppo A refugee woman carries her belongings near the Turkish border gate on February 6. Hide Caption 8 of 17 Photos: Syrian refugees flee embattled city of Aleppo Refugee children sit on a car near the Turkish border crossing on February 6. Hide Caption 9 of 17 Photos: Syrian refugees flee embattled city of Aleppo A refugee warms himself at a bonfire near the Turkish border on February 6. Hide Caption 10 of 17 Photos: Syrian refugees flee embattled city of Aleppo Refugees brave the cold and rain as they arrive at the Turkish border on February 6. Hide Caption 11 of 17 Photos: Syrian refugees flee embattled city of Aleppo A child tries to climb over a fence near the Turkish border as Syrians fleeing the northern city of Aleppo wait on February 6. Hide Caption 12 of 17 Photos: Syrian refugees flee embattled city of Aleppo A young refugee carrying belongings arrives at the Turkish border on February 6. Hide Caption 13 of 17 Photos: Syrian refugees flee embattled city of Aleppo Refugees arrive near the Turkish border as they flee the city of Aleppo on February 6. Hide Caption 14 of 17 Photos: Syrian refugees flee embattled city of Aleppo A child carries his belongings as Syrian refugees arrive at the Turkish border on February 6. Hide Caption 15 of 17 Photos: Syrian refugees flee embattled city of Aleppo A child sleeps as Syrians fleeing the northern embattled city of Aleppo wait near the Turkish border on February 5, 2016. Hide Caption 16 of 17 Photos: Syrian refugees flee embattled city of Aleppo A Syrian teenager and a child look on near the Turkish border on February 5. Hide Caption 17 of 17

But they lacked proper political leadership. The Syrian opposition leaders became a bit of a five-star farce, lounging around in luxury hotels. They bickered amongst themselves so fecklessly that it became hard to keep track who was actually their leader.

These endless squabbles began to frustrate international supporters. Then 2013 came and the Assad regime was still holding on, even crossing President Barack Obama 's famous red line of chemical weapon use, all to no military consequence. Meanwhile, radicals began to seep into the rebel ranks. First it was the Nusra Front or al-Qaeda in Syria, considered by Hillary Clinton back then to be a simple extension of al-Qaeda in Iraq. Nusra were terrorists to U.S. eyes, but to many Sunni Syrians they were the ones doing the real fighting against the regime. There was little western support -- but there was Nusra.

The group continued to grow, joining a rebel coalition that saw tremendous success against the regime in late 2014 and 2015: the Army of Conquest. This alliance contained moderates, but also radicals, and al-Qaeda . It took huge swathes of territory from the Assad regime, prompting the Russian intervention. But it lacked western support as, beneath it all, al-Qaeda was doing a lot of the fighting.

Photos: Syrian girls: What we want to be Photographer Meredith Hutchison of the International Rescue Committee asked Syrian girls who are refugees in Jordan what they want to be when they grow up. Here's what they said and how they picture themselves in the future.



Rama, 13. Future job: Doctor -- "Walking down the street as a young girl in Syria or Jordan, I encountered many people suffering -- sick or injured -- and I always wanted to have the power and skills to help them. Now as a great physician in my community, I have that ability. Easing someone's pain is the most rewarding aspect of my job."

Hide Caption 1 of 10 Photos: Syrian girls: What we want to be Muntaha, 12. Future job: Photographer – "Since I was a young girl, I loved taking people's photographs. I loved going to different events and documenting what was happening -- both the good and the bad. Now, as a professional photographer, I use my images to inspire hope in others -- to encourage love and understanding." Hide Caption 2 of 10 Photos: Syrian girls: What we want to be Hiba, 9. Future job: Pediatrician – "I have always wanted to help children, and this is what drove me to be a pediatrician. I am kind and loving, and therefore an excellent doctor that children can trust." Hide Caption 3 of 10 Photos: Syrian girls: What we want to be Haja, 12. Future job: Astronaut – "Ever since we studied the solar system in primary school, I have wanted to be an astronaut. I would imagine myself up in the sky discovering new things. In this society my path was not easy -- many people told me a girl can't become an astronaut. Now I have achieved my goals. I would tell young girls with aspirations not to be afraid." Hide Caption 4 of 10 Photos: Syrian girls: What we want to be Fatima, 12. Future job: Teacher – "In this image, it is the early morning, and I am waiting in my classroom for my students to arrive. I teach younger children to read and write Arabic. I am strict, but I go out of my way to gently help those students who are having difficulties." Hide Caption 5 of 10 Photos: Syrian girls: What we want to be Mona, 10. Future job: Doctor – "In this image, I am in the future and a well-respected physician at a major hospital in Syria. I am asking a patient about her pain and helping her to get better. My mother was born and raised in a village and didn't go to school, but as a young girl, I had the opportunity to learn and grow into a great doctor." Hide Caption 6 of 10 Photos: Syrian girls: What we want to be Amani, 10. Future job: Pilot – "I love planes. Even before I had ever been on a plane, I knew I wanted to be a pilot. Now, not only do I get to live my dream, but I also get to help people travel, to see the world and discover new places." Hide Caption 7 of 10 Photos: Syrian girls: What we want to be Merwa, 13. Future job: Artist – "In this image, I am a popular painter, working on a landscape in oils. My hope is that my artwork inspires peace in the world and encourages people to be kind to one another." Hide Caption 8 of 10 Photos: Syrian girls: What we want to be Wissam, 15. Future job: Pharmacist – "Our neighbor in Syria had a pharmacy, and when I was younger I used to go next door and help. As the war started, I watched this pharmacist help the injured. When I saw this, I knew that this was an important job and what I wanted to do." Hide Caption 9 of 10 Photos: Syrian girls: What we want to be Nour, 16. Future job: Lawyer – "I want violence against women to end. I want women to be able to make decisions for the community ... without fear. I want our society to open up and give space for women to be whoever they want to be. ... Now I am a respected lawyer working on women's rights and defending women who are victims of domestic violence." Hide Caption 10 of 10

Simultaneously, in 2014, ISIS spread across the border from Iraq. Also Sunni, their initial benevolence toward the communities they swallowed was quickly usurped by a ghastly brutality that made them the international menace they now are. The U.S. found itself drawn into an air war against them, and found that the Kurdish forces to ISIS's north-east were actually the most effective ground force they could use against them.

Separately, the U.S. tried to train and equip moderate Sunni Syrians, and put aside half a billion dollars to do so. But the program's lengthy screening process and ban on candidates fighting the regime sapped its appeal, until only 54 recruits emerged and the program ended. The moderate Sunni Syrian rebels continued to play their Gulf and western sponsors off against each other, and still resolutely failed to unite.

Where are we now: Opportunity for radicals

The Kurds are still America's main ally in the fight against ISIS. But now they are advancing against the very forces the US have courted to fight the Assad regime: the moderate Sunni Syrian rebels.

These moderates are now perhaps fatally weakened in a northern battlefield where the Kurds grow stronger daily; where the Assad regime continues to advance with Russian air support, trapping many anti-regime fighters inside the ruins of Aleppo's east; and where even a beleaguered ISIS is using the chaos to retake some land.

Now that Russia has joined the war, the geopolitical implications of this mess are staggering. Turkey still supports the moderate Syrian rebels, and has hinted that if Azaz falls to the Kurds it may intervene. But in doing so, it would intervene against America's key ally in the war against ISIS: the Syrian Kurds. Turkey and the U.S. are NATO allies -- and the prospect of their open disagreement must leave the Kremlin salivating.

This torturous scattergram of alliances leaves one group almost unrepresented on the battlefield, the greatest victims of the war -- ordinary Syrian Sunnis. ISIS claim to represent them; al-Qaeda does a lot of the fighting to protect their traditional areas. But the moderate forces who began their rebellion in the north are in disarray.

Now, as we have seen time and time again, radicals may well step into the void.