US and Israeli officials have voiced grave concerns about what appears be an emerging land bridge of fighting groups loyal to Tehran stretching from Iran’s Zagros Mountains all the way to the borders of Israel — but seem powerless in their attempts to stop it. Iran appears to be using these men in ever more creative ways, in an ongoing sectarian and geopolitical war that pits pro-Iranian Shiite countries and organizations against a Saudi-led bloc of conservative Sunni governments backed by the US.

Others worry that building up a multinational network of highly trained and experienced combatants could have lasting consequences for a region already awash in arms, extremism, and overlapping conflicts. “What will happen with all these Iraqis, Afghans, Pakistanis fighting in Syria? Many will go home,” said Karmon. “But 500 or 1,000 is all it takes to organize a terrorist network. It will be a future threat. Their role is now strategic.”

The Iranian training of militia fighters seems to be accelerating, with fresh recruits and veterans of past training missions planning trips to Iran this year, according to the fighters themselves. Recent recruits described being trained in the use of explosively formed penetrators (EFPs), which can pierce the armor of military vehicles and were used extensively against US forces during the occupation of Iraq a decade ago.