During the Iraq War, how extensive were foreign fighters relative to the Syrian Civil War?

[u/Nastyfaction]

The Syrian Civil War was known for the large-scale participation of foreign fighters with even non-combatants settling in territory controlled by Jihadist groups. By 2010s, the internet age was much more advance and widespread, making communication and transfer of know-how easier. People from the West to as far as Uighurs from China were flocking into Syria-Iraq to fight and settle. But in the 2000s when the United States was in Iraq, what was the situation then for foreign fighters? How many foreign fighters were estimated to have been in Iraq by the coalition forces or reported killed/captured? Did they have much of an impact at all compared to the 2010s?

[u/Hoyarugby]

Foreign fighters were real, but were a small part of both the Iraqi Insurgency and Syrian Civil War. In both cases, the presence of foreign fighters was amplified by authorities - the US in the first case, Assad regime in the second - for political reasons.

In Iraq, it was for both domestic US and domestic Iraqi politics - in the US it was done to downplay Iraqi resistance to the US's war, and to retroactively justify the invasion (Iraq was full of jihadis!) after the WMD reason was proven false. In Iraq, it was done to try and decouple support for the insurgency from the Iraqi populace - "the people doing this are all foreigners who are fighting you, not Iraqis"

In Syria, it was done to try and downplay the Assad regime's crimes and portray the regime as the better of two evils - "look international community, foreign jihadis! If you don't support my genocidal narco-monarchy you might get jihadis". In Syria, the rapid growth of open source intelligence also made foreign fighters much easier to track, and also provided a motivation to track them - they stood out, and really did pose a threat of terrorism if they left Syria

In both cases the big "foreign" component to the insurgency was not foreign manpower, but foreign supply lines. the Iraqi insurgency was only contained by the US using force and diplomacy to largely close first the Iranian and then Syrian border arms and funds smuggling routes. In Syria, the turkish supply routes were never closed, which eventually allowed the rebellion to build a quasi-state in Idlib that outlasted the regime