They stopped Islamic State dead in its tracks in northern Syria, driving the group famously from Kobane, then Manbij, then finally liberating the IS "capital" Raqqa as the spearhead of US-led international coalition forces.
But thanks to their success, the Syrian Democratic Forces say they are now stuck with hundreds, possibly thousands, of foreign IS fighters captured on the battlefield - and not one of their European allies has said it is willing to take any back.
Gabriel Kino, an SDF spokesman, told MEE that they have sent letters to many Western countries to convince them to take action.
"The SDF so far has captured thousands of IS militants, both Syrian or foreign fighters that joined IS during the last few years," he said, speaking from an SDF base in Ain al-Issa.
"We are conducting investigations and trying to get as much information as possible from them to use in the war against IS, which is still ongoing [in Deir Ezzor]".
But, he says, they have no clear view what they are going to do with them.
"We send letters to ask foreign governments to take those foreign fighters back and put them on trial, but so far we have not received any responses," Kino said.
His comments come after statements from the UK and France that the best option for Europe is that the IS fighters never come back.
In October the French defence minister, Florence Parly, said she did not want French IS fighters to return, and it would be "best" for them to die in the field.
Weeks later Parly's British counterpart, Gavin Williamson, said "a dead terrorist can't cause any harm to Britain", and that British IS fighters should be hunted down and killed, instead of returning home.
Some British citizens in Syria have already been stripped of their citizenship on the grounds of alleged links to extremist groups, although exact numbers are unknown.
Attitudes appear to change in Britain however when high-profile IS are captured. Calls are growing for two captured members of the notorious British "Beatles" group, which murdered and tortured foreign captives, be tried in a British court.
Kino said the SDF had no way of knowing how to deal with the attitudes of European states.
"They are not Syrians or have a Syrian nationality for us to prosecute them and put them on trial here," he said.
"What the SDF needs is more cooperation from the international coalition and international community to deal with this problem that affects our work," he added.
"It’s not only the obligation of the SDF to deal with this problem," he said.
"It should be collaborative work with different law enforcement agencies from different countries in order to deal with foreign fighters," he said.
Sadet Karabulut, a Dutch Socialist Party MP, told MEE that the reason was simple: "Countries are reluctant because they are afraid of attacks in Europe."
"You don’t know who you are bringing in," she added, "jihadists that possibly were responsible for crimes against humanity or extremists that cannot tolerate people who think differently."
She and another MP, Lilianne Ploumen, last month submitted a motion to parliament which would ensure that Dutch Islamic State suspects held without trial in Syria or Iraq were brought to justice according to international law, especially those accused of involvement in genocide.
"Ideally, a solution should be found, so that all IS militants will be sentenced and victims are compensated," she added.
A spokesperson for the UK's foreign office would not comment on whether the government wished for suspected IS fighters to be returned home, but said: "Everyone who returns from taking part in the conflict in Syria or Iraq must expect to be investigated by the police to determine if they have committed criminal offences, and to ensure that they do not pose a threat to our national security. Where appropriate, they will be prosecuted."
Distracted in Afrin
And the Kurds say the Turkish operation "Olive Branch" in the region of Afrin, launched on 20 January, forces the Kurds to neglect the war against IS, with the SDF now redeploying 1,700 fighters from the IS fight to Afrin.
"We, the SDF, have protected the world from IS terrorism, and there are hundreds of them [IS foreign fighters] in our prisons, including in Afrin, but now the Turkish army is attacking us [in Afrin]," Nouri Mahmoud, the spokesman of the People’s Protection Units (YPG) told MEE, speaking from a base near Qamishli in north-east Syria.
"It is possible they could escape from prisons [in Afrin] in these attacks, and they could pose a serious threat," he added.
"Many European countries don't want to take those terrorists back," he added.
"Since they are international terrorists and they don’t want them back, we have to find a way to deal with them within the federation of northern Syria," he said.
But the lack of international assistance in dealing with this judicial issue was now helping IS on the ground, he said.
"Turkish attacks on us help revive IS in Deir-Ezzor again, and the international community should take a stance," he added, referring to the resurgence of IS in some parts of eastern Syria.
Moreover, the Kurdish-led SDF forces say they are suffering from a lack of resources, and are stretched to the limit holding the thousands of foreign and local IS members and their families. This is increasingly worrying US officials, it has been reported.
A few weeks ago, the US confirmed that "less than" five fighters recently escaped from SDF detention, highlighting these concerns.
US officials have been repeatedly stressing that origin countries need to come forward to claim their fighters.
"The important thing is that the countries of origin keep responsibility for them," said the US defence secretary, James Mattis, last month.
"How they carry out that responsibility - there's a dozen different diplomatic legal or whatever ways, I suppose.
"There is not a one way forward... It's being worked. There's a number of things going on already to have some of them being repatriated to certain locations but right now we do not have that one fully resolved," he concluded.
Unlike foreign fighters, their families, including their wives and children, are held in camps for displaced Syrians and refugees, like in the Roj camp in Hasakah province, or the Ain al Issa camp near Raqqa city.
It is increasingly difficult to get permission to interview these IS families after local authorities made a decision not to allow journalists to visit them, although from time to time exceptions are made.
"Families do not have anything to do with the actual crimes committed by foreign fighters, but they are still IS and they are currently held in camps," said Kino, the SDF spokesperson.
"This process takes a lot of our resources, regarding the way we deal with them. We need more support and more cooperation with the international community to deal with this problem," he added.
Shouldering the burden
Kurdish officials are keen to stress that they can’t shoulder the burden alone.
"We are in a different situation, facing war, having no resources, for holding them for years in prisons. They are from 40 countries. This is very difficult for us," Abdulkarim Omer, the Foreign Affairs head of the Jazira canton told MEE.
"We can’t do all of this work. That’s why European countries should take them back to their own country, and sentence them there."
According to the YPG spokesperson, the alternative is to establish an international court in Syria.
"The international community must establish courts in northern Syria for the trial, and the whole world should participate in it," Nouri said.
Shahoz Hassan, the co-chair of the Kurdish Democratic Union Party (PYD), said they can put them on trial in local courts if the Western countries will not accept them.
"For foreigners who joined IS, it would be better to put them on trial in their own countries if they accept them," he said.
"If they do not accept them, we will put them on trial according to our own laws," he said.
Wives and children
Andre Seebregts, a Dutch defence lawyer representing several wives of IS fighters now stuck in Syria, criticised his government’s policy on this issue.
"I understand that Russia and Indonesia have picked up the women, but I have not heard of EU countries picking up the families," he said.
However, he said that the Dutch government policy is that they will not help the children and wives in Syria, unless they make contact with Dutch diplomatic representation in either Turkey or Iraq, as the Netherlands has no consular presence in Syria.
"This policy of the Dutch government is not wise," he said.
"It would be better to let them return in an organised way, then you know who they are, and when they will arrive," he said.
"You cannot say those children are a danger for the Dutch society, I doubt it is too late to integrate them," he said, referring to a report from the Dutch intelligence agency this week which suggested that IS children and wives pose a threat.
He said that the Dutch government needs to find a solution to the issue of IS wives and children still in Syria.
"This is not a solution to keep them in Syria. At some point they will have to come back."
Back in northern Syria, the SDF's US allies say the world owes a debt to Kurdish fighters in Syria, and should do all it can to help them.
"We firmly believe nations whose citizens are detained as Daesh fighters must come forward, claim their citizens, and ensure they face justice in their countries of origin," said Thomas Veale, a colonel and spokesman for the US-led coalition against IS.
"Our Iraqi and Syrian Democratic Forces partners have detained many Daesh terrorists, benefiting the world by removing them from the battlefield and preventing them from transiting the region and posing threats to our homelands," he added.