Recent media reports on the journey taken by three members of the Dutch biker gang No Surrender to support Kurdish fighters against the Islamic State in Iraq and Syria (ISIS) point to an under-explored phenomenon - that of foreign fighters taking up arms against ISIS.

Much attention has focused on Westerners supporting the ISIS; much less has been put on a small but growing number of Westerners joining the fight against the Islamic militant organisation.

Understanding this development may throw some light on how the conflict in Syria and Iraq might unfold.

What precisely motivated the Dutch biker gang other than sympathy for the Kurds is unknown.

There are some suggestions that the biker gang's involvement had been purely to support aid distribution - until ISIS' recent gains and atrocities against minorities such as the Yazidis and Christians made them take up arms. "You can't stay sitting on your couch," said one of the gang members when interviewed by the media.

Who are these anti-ISIS fighters?

A small number of individuals from the Kurdish diaspora in Europe are known to have returned to support their brethren, fighting with the YPG (the Kurdish People's Protection Units in Syria) or the peshmerga in Iraq. Many more are involved through fund- raising in Europe.

Kurdish activists in Europe and Britain say that ISIS' successes have caused more Kurds to make the journey to fight as it threatens the existence of the Kurdish people. It also puts at risk the dream of an autonomous Kurdish entity spanning the Kurdish areas in Syria, Iraq and Turkey.

Syrian and Iraqi Christians have also begun to mobilise as regions formerly untouched by the conflict are now falling under the shadow of ISIS.

Their situation - outmanned and largely outgunned - has not gone unheeded by members of the Syriac Christian diaspora. A Swiss of Syrian Christian origin, Johan Cosar, has been fighting for the Syriac Military Council (known as the MFS), a Christian militia group active in the mainly Kurdish region of North-east Syria.

Cosar, a former soldier in the Swiss army, has imparted some of his military training to MFS fighters, and appears himself to have taken part in fighting against ISIS in joint MFS operations with the YPG.

Reports suggest that there are several others like Cosar. A key motivating factor appears to be empathy for co-religionists. Another Swiss national active in Iraq, interviewed by the media, commented: "Someone has to take action to prevent the disappearance of Christians."

Three Americans, all with some military experience, are known to be fighting with the YPG in Syria. They appear to see themselves as responding to the call of moral duty. As Jordan Matson, the most prominent of the Americans with the YPG, states in an interview: "I can't just stand by (while men, women and children are killed)."

There is also some suggestion of rootlessness or psychological displacement. The Americans in question appear to be not deeply rooted in the United States and may be searching for some sort of cause.

So far, none of the groups fighting ISIS or other jihadist elements in Syria or Iraq has issued a general call to arms for foreign fighters to come to their aid. Spokesmen for the Kurdish regional government in Iraq and peshmerga officials have indicated that what are needed are arms, not manpower.

The network to bring such fighters in, however, is clearly in place.

The Americans fighting with the YPG appear to have first got in touch with YPG elements on social media. There are scattered indications of a similar underground network that assisted Europeans of Syriac Christian origin in making the journey to Iraq and Syria.

Also in place is a rudimentary "cheerleading" element. One of the Americans fighting with the YPG - Matson - has taken up the role of answering questions from others (including, by his own account, hundreds of individuals from the West) considering going to join the anti-ISIS fight.

Some commentators have made comparisons to the "International Brigades" of the Spanish Civil War (1936-39) - individuals largely drawn from various parts of the Western world who fought against fascist forces.

These comparisons are for the time being fanciful. There are, however, intriguing signs that some individuals without any direct religious or ethnic affiliation to the groups under threat by ISIS may consider joining the fight. Recent media reports suggest that citizens of Greece and Turkey (including individuals of non-Kurdish origin) have joined the YPG, motivated by the threat that ISIS poses to greater humanity.

If ISIS gains ground in a manner which imminently threatens genocide of Kurdish and Christian populations in Iraq and Syria, increasing numbers from the West may consider it their duty to fight for their ethnic brethren or co-religionists.

Already, some websites with a fundamentalist Christian orientation have started to talk of a crusade or "reverse jihad" against ISIS. The related, longer-term possibility is that the clash against ISIS may acquire overtones of a larger inter-religious conflict.

There is also the question of the fate that awaits these anti- ISIS foreign fighters (should they survive) if and when they attempt to return to their home countries.

US officials have stated that it is illegal for an American to fight for Syrian militia. But various European Union states appear - for the time being at least - to be prepared to look the other way. The Dutch authorities have, for example, suggested that there is nothing to prevent Dutch Kurds from joining the anti-ISIS fight, and that such individuals do not face prosecution on their return to the Netherlands.

Technically, however, it remains illegal to join the Kurdistan Workers' Party, the PKK, which remains a proscribed terrorist organisation, and which has close links to the YPG.

Unless the authorities make clear their stance on their citizens joining the anti-ISIS conflict, unofficial boots on the ground may arrive in greater numbers, whether governments like it or not.

stopinion@sph.com.sg

The writer is senior fellow and deputy head of the Centre of Excellence for National Security, a constituent unit of the S. Rajaratnam School of International Studies, Nanyang Technological University.