No one really dwells on the question of why so many young men from Europe, Canada, Australia, even China, are going to fight in Syria and Iraq with the so-called Islamic State (Isis), or with other Islamist militias. The New York Times recently published a map showing which countries the foreign volunteers come from. The numbers are slippery and often contradictory, but the foreign presence in Syria and Iraq is reckoned at around 17,000 fighters. The biggest contingents are from Chechnya and the North Caucasus (around 9000) and Turkey (1000). There are also 400 from Kosovo. But 1900 come from Western Europe (700 from France, 340 from Britain, 60 from Ireland), 100 from the US, and between 50 and 100 from Australia.

The prevailing view is that these volunteers are marginalised fanatics: in other words, they’re ‘crazy’. Madness has been used to explain everyone from Caligula to Hitler, Idi Amin and Saddam Hussein and any other leader or dictator who has been either defeated or marked for defeat. But it is an explanation that explains nothing, and which rather indicates that we are incapable of explaining the phenomenon. We need to be extremely careful with the way we define other people: no one defines himself as a ‘terrorist’ (just as no one defines himself as a ‘populist’). During the Second World War, the Germans called the maquisards ‘terrorists’, but after the Allied victory no one called them that any more. The French called the FLN in Algeria ‘terrorists’, but after independence no one used the term, simply because the FLN had won. No one called Begin or Ho Chi Min a ‘terrorist’, because they too imposed a victor’s narrative.

The conclusions drawn in An Economic Analysis of the Financial Records of al-Qaida in Iraq, prepared by the Rand Corporation in 2010, apply to Isis too, the institute says. First, financial gain is not the principal motive driving people to join Islamist militias: a fighter earns a lot less than the regional average, while his chances of dying are a lot higher. Second, the terrorists have higher than expected levels of education and wealth, ‘which weakens theories explaining individual participation in militancy as being due to financial deprivation, mental instability or poor education’: the volunteers, according to the Rand Corporation, are not marginalised, crazy or poor.

The new international brigades are a phenomenon that needs to be taken seriously, and raises a serious question. The first modern ‘foreign volunteers’ were those who in the early 1800s went to fight and die for the independence of Christian Greece from the Islamic Ottoman Empire, including Santorre di Santarosa (who died on Sphacteria in 1825) and Byron (who died at Missolonghi in 1824). Garibaldi was known as the ‘hero of two worlds’ because he fought in Brasil, Uruguay, Italy and France (in 1870-71, against the Prussians). They all embodied the words of Emile Barrault, a follower of Saint-Simon who told Garibaldi in 1833 that ‘a man who, making himself cosmopolitan, adopts humanity as his fatherland and offers his sword and his blood to all peoples who struggle against tyranny, is more than a soldier; he is a hero.’

The tradition was revived in the 20th century by the International Brigades – anarchist, republican, communist – in the Spanish Civil War, but after the Second World War it came to an end. No European volunteers went to fight in Vietnam or Apartheid South Africa.

It resumed in the 1990s with the war in Bosnia and then in Afghanistan, and now the enlistment with Isis which is getting so much attention. But there is an enormous difference in the motivation of these new international volunteers. It is no longer national liberation or class solidarity, but a new religious irredentism that wants to free itself from the yoke of the infidels. Isis’s European volunteers could adopt Barrault’s slogan if they replaced ‘humanity’ with ‘Wahabbi Islam’.

The question is, how has it come about that young Europeans are no longer prepared to sacrifice themselves for humanitarian, patriotic or socialist reasons, but are for religious ones? What have we done to them to bring them to this point? What’s infuriating about the dominant discourse on Islamic fundamentalism, especially in Europe, is that it glides over structural causes and social alienation, and reduces everything to the implausible and useless category of ‘insanity and fanaticism’.

That Isis are far from insane is demonstrated by the fact that, with two public beheadings, this motley crew managed to get itself recognised as the main enemy of the world’s biggest superpower.

A German version of this piece was published by 'Die Tageszeitung'.