A flag showing Syrian President Bashar al-Assad flutters near damaged buildings in Adra northeast of the capital Damascus on Sept. 25, 2014. (LOUAI BESHARALOUAI BESHARA/AFP/Getty Images)

Over the past year, the phrase "foreign fighter" has become part of the standard vocabulary for Middle East affairs, frequently used to describe the young men and women who traveled to Syria to join a bloody civil war. To a large extent, "foreign fighter" has become synonymous with the Sunni Muslims who join groups like the Islamic State or the al-Nusra Front.

Sunni Muslims are not the only foreign fighters in Syria, however. In a new report for the Washington Institute, titled "The Shiite jihad in Syria and its regional effects," Philip Smyth points to the growing yet largely ignored influence of foreign Shiite Islamists who have joined the fighting.

Smyth, a researcher with the University of Maryland, argues that Shiite foreign fighters have in fact played an overlooked role in the conflict, coming to the aid of Bashar al-Assad's government when it looked on the verge of collapse and helping the Syrian president win back vast amounts of land. "Foreign fighters were central to both the rapid turnaround on the front and the Assad regime’s continued survival," Smyth writes.

While the most notable groups to enter the fray are the Lebanese Hezbollah and a number of different Iraqi Shiite militias, the Washington Institute's report also points to more exotic Shiite foreign fighters, including a number from Afghanistan. There also appeared to be a number of cases of Westerners joining the Shiite Islamist cause in Syria: Last year, a Michigan man was arrested while attempting to join Hezbollah to fight in Syria, for example. (There have also been other high-profile accounts of Westerners going to fight for Assad – two L.A. gang members went last year – but their motivations were not religious).

The Shiite foreign fighters who head to Syria are motivated by a variety of factors, including a desire to protect the Shiite holy sites in the country (most notably the Sayyeda Zainab shrine in Damascus), protect vulnerable Shiite civilians, and out of a sense of duty to Assad's Alawite regime. However, the Washington Institute's report also argues that much of the influx of Shiite fighters has been orchestrated by Iran, the regional Shiite power, which has been directly involved and sent its own troops to oversee the fighting. "In a wider political sense, the real victor of the Syrian war and in Iraq has been Iran, a triumph for which the Islamic Republic has its militia forces to thank," Smyth writes.

Some of the efforts by Shiite Islamist groups echoes that done by Sunni groups. For example, Shiite Islamist groups use social media to recruit potential fighters: Potential fighters might be recruited directly on a social network like Facebook or directed to a phone number where they are given a brief interview.

"It's both shocking and unnerving that while ISIS pages have been effectively targeted by Facebook, [pages related to Shiite groups like Hezbollah] have not only been left up, but more have been created," Smyth writes in an e-mail to The Post, using an acronym to refer to the Islamic State. "It certainly demonstrates a myopia in terms of focusing on extremist groups."

Smyth estimates there are around 5,000-7,000 Lebanese Hezbollah fighters in Syria at any one time and an additional 5,000-10,000 Iraqi Shiite fighters in the country, though he emphasizes that finding accurate figures for Shiite foreign fighters in the country is exceptionally hard. The number of Shiite Afghan fighters is smaller, probably in the hundreds, Smyth says, and there are probably a few thousand members of Iran's Islamic Revolutionary Guard Corps there.

These figures are comparable, though lower, than the number of foreign fighters estimated to have joined Sunni Islamist groups in Syria and Iraq, which is estimated to be around 20,000 in total.

To anyone closely following Syria's crisis, the presence of Hezbollah and other Shiite groups in the country won't come as a surprise (one Israeli study from over a year ago suggested Shiite fighters actually outnumbered Sunni fighters). However, the Washington Institute's report emphasizes that Western fears about Sunni foreign fighters, largely driven by concerns about the threat posed by their return, may have overshadowed important knock-on effects of Shiite foreign fighters in Syria.

Those effects are already being felt in Lebanon and Iraq and they could spread farther. And while these Shiite fighters are often fighting against the Islamic State, a group the United States seeks to destroy, they definitely do not see themselves as being on the same side as the United States or its partners.