This mixture of financial and ideological incentives was seen again later in the Syria war, when Iran created two new brigades, the Liwa Fatemiyoun and the Liwa Zainebiyoun, each consisting of roughly 10,000 troops each, although estimates vary.
These were mostly Afghan and Pakistani Shiites living in Iran, often in refugee communities, that were trained and paid by Tehran to fight in Syria. As with Hezbollah, there was a religious motivation at play as these fighters were initially sent to guard Shiite shrines in Syria before later being deployed to the front line.
But there was a financial incentive. Many were promised Iranian citizenship in exchange for their service, as well as a salary that those living as refugees would not have accessed otherwise.
Importantly, the ideological motivations had not been sufficient to motivate these soldiers to travel to Syria independently, they needed the financial motivation too.
Though Tehran would not like to describe these forces as mercenaries, the logic for Iran using them does fit the pattern seen elsewhere.
In the case of Hezbollah, the Lebanese militia provided valuable skills that the Iranian military lacked, helping to train al-Assad's forces in the kind of urban warfare skills it had gained in fighting Israel.
Hezbollah and Syrian flags flutter on a military vehicle in Western Qalamoun, Syria, Aug, 28, 2017.
In the case of the Iraqi, Afghani, and Pakistani Shiite fighters, their deployment limited the number of Iranians killed in Syria. Again, numbers are hard to pin down, but it is believed approximately 2,300 Iranian soldiers died in the war, but a further 6,400 Iraqi, Afghani and Pakistani fighters were killed, plus up to 2,000 from Hezbollah.
Without these foreign fighters, Iran might have suffered up to 10,000 casualties, a figure far more likely to provoke a domestic backlash and evidently something Tehran wished to avoid.
Iran has a huge military, of over 600,000 active personnel, yet only deployed 15,000 or so of them at a time to Syria. It deployed far more non-Iranian troops who, in many ways, resembled mercenaries.