Today, the fight against the Islamic State terrorist group has become a top strategic priority of the United States and its allies in the region. In turn, the efforts of Washington and Middle Eastern partners have begun to pay real dividends, with recent months seeing a significant rollback the group's self-declared "caliphate" in Iraq and Syria. But lurking in the background of the current counterterrorism fight is another, and potentially even more significant, long-term threat.

Since its rise to prominence in 2014, one of the Islamic State's most striking - and formidable - features has been its ability to inspire and attract disaffected extremists to its cause. Experts estimate that, to date, the group has drawn some 32,000 radicals from the Middle East, Europe, Africa, and beyond to its nascent state.

Thanks to the influx of these foreign extremists, Syria has steadily transformed into a training ground for today's terrorists and a crucible for a coming wave of extremism. It is, in other words, the new Afghanistan - albeit on a significantly larger scale. The number of foreign fighters in the Islamic State is more than one-and-a-half times the size of the total contingent that joined the Afghan jihad against the Soviets in the entire decade between 1979 and 1989.

As impressive as it is, however, the size of the Islamic State's jihadi contingent is dwarfed by a second, far less well understood foreign fighter stream: the Shi'ite militias and fighters that have been mobilized over the past two years by the Islamic Republic of Iran.

Just how big is this cohort? All indications suggest that it is large, and getting larger still.

This spring, the government of beleaguered Syrian dictator Bashar al-Assad reportedly asked Iran to take charge of organizing and paying the tens of thousands of Shi'ite irregulars now fighting alongside official Syrian, Russian and Iranian forces. The size of this force - which is drawn from Yemeni, Pakistani and Afghan Shi'a, as well as assorted Iraqi militias - now numbers "more than 50,000 militants," according to one anonymous Syrian official.

Observers, however, believe that this could be just the tip of the iceberg. For example, Nader Uskowi, a specialist in Iranian military affairs at the Washington Institute for Near East Policy who previously served as an advisor to the United States Central Command, estimates that the number of fighters that have been mobilized to date by Iran could be as large as 200,000. If that figure is anywhere close to accurate, it would mean that Iran's foreign legion is only slightly smaller than the entire army of Saudi Arabia, equal in size to Afghanistan's military, and larger than the total troop strength of the Kingdom of Morocco.

It is also a force that represents a grave and growing threat to the Middle East. Today, as the tide of the war begins to turn decisively against ISIS, regional governments are preoccupied with the inevitable return of "alumni" of the Syria conflict to their home countries and regions, and the growing potential for domestic instability as a result.

But where this Sunni contingent has been self-generated and self-organizing, its Shi'ite counterpart is both directed and sustained by the Islamic Republic. And in the future, Iran's leaders may well harness their new expeditionary force beyond the Syrian battlefield, using it as a tool to pursue other geopolitical objectives and target regional rivals.

If they do, the nations of the Middle East will be faced with a new, and formidable, asymmetric adversary. So, too, will the United States.