Since the end of 2012, social media has been critical in recruiting Shiite foreign fighters in Syria and Iraq. Stashed away in images on Facebook pages, profiles and blogs, one can find phone numbers and websites for establishing contact between recruiters and potential fighters. According to Philip Smyth (researcher at the University of Maryland), by May 11, 2013, a sign-up page known as ValieAmr (now offline) had registered some 3,253 Shia volunteers ready to fight. What are their motives and what are the possible consequences for Europe?

According to Al-Monitor the latest estimates on the number of foreign Shiite fighters in Syria are between 10,000 and 15,000. The Meir Intelligence and Terrorism Information Centre estimates the number of Shiite fighters in Syria between 7,000 and 8,000. And, according to the Terrorism Research and Analysis Consortium (TRAC), the London-based Arabic newspaper Al-Sharq al-Awsat has given an estimate of 40,000 Shiite fighters. Numbers of Shiite fighters in Iraq are not available.

Although the estimated number of foreign Shiite fighters in Syria varies, they often claim to serve the same purpose: defending Shiite shrines and believers. A statement released in December 2013 by the Popular Committee for the Moblization to Defend Sayyeda Zainab (Lijna al-Tabia al-Shabiyah an al-Sayyeda Zainab) issued this argument about the legitimacy of the Shiite jihad in Syria:

"It is obvious that the goal of going to Syria is to defend the Shiites and the sect of Ahl al-Bait and Islamic shrines and the resistance because the takfiri groups [Muslims who excommunicate other Muslims] in Syria have been targeting our sect and our shrines in a clear and direct way. (…) This is a defensive war and defensive wars do not require permission from anyone. (…) The Jihad in Syria has been overseen by a legitimate ruler who is the veli-e faqih [religious and political authority, in this case referring directly to Iranian Supreme Leader Ayatollah Khamenei]."

According to the concept op the veli-e faqih, introduced by Ayatollah Khomeini during the Iranian Revolution (1978-79), the Shiite jihad "must expose and overthrow tyrannical rulers and rouse the people so that the universal movement of all alert Muslims can establish Islamic government in place of tyrannical regimes". If the oppressive and deviant rulers do not listen to the wishes of a Shia movement, it will be the duty of the Shiites – according to Khomeini’s work – to engage in an armed jihad.

This narrative of the Shiite jihad has now mobilized thousands of Shiite fighters in opposition to their Sunni (extremist) foes in Syria and Iraq who are destroying their holy shrines and killing Shiite believers.

In Europe there are no holy shrines that need to be defended in line with this ideology. But how likely is the chance that the Shiite Muslims in European countries who are willing to travel to Syria and Iraq to fight for Iranian-backed Shiite groups, will become a new threat to the Western society – by returning as battle-hardened Shiite fighters?

According to Samuel Westrop, a Senior Fellow at the Gatestone Institute and Associate Director at the Institute for Middle Eastern Democracy, Iran attempts to recruit Western Shiites to stop the expansion of IS in the Middle East. An example is the English branch of the Ahlul Bayt World Assembly, an Iranian Shiite clerical organization, that has published in 2014 an "Urgent Call" to "take arms in defense of Iraq, its citizens, and the holy shrines".

Besides that, the prospect of returning Shiite fighters increases the possibility of sectarian tensions between Shiites and Sunnis in European cities, as these returning fighters might - in addition to battleground experience - also bring back a strong sectarian ideology. Although the discussion in Western media has focused mainly on the threat posed by returning Sunni extremist fighters, the prospect of returning Shiite fighters is also alarming. Let us be aware of the ‘other’ jihad and not just focus on the term ‘jihad’ as a synonym for Sunni jihadists.