Article in Lawfare blog about foreign fighters in Ukraine with references to the participation of foreign fighters in other conflicts.
Basic message, its not a good idea for a number of reasons.
The military record of foreign fighters is mixed. In many cases, they are zealous but untrained and do not come well-armed. They are eager for combat and (in the case of jihadists) martyrdom, but overall adding little to the combat power of their fellow fighters, especially in cases, like Afghanistan in the 1980s and Ukraine today, where there is plenty of manpower. Most end up as cannon fodder. In some instances, however, the fighters themselves come with skills—one report indicates that American and British special operations forces veterans are preparing to deploy to Ukraine—and this cohort can be far more useful. And in many other instances, foreign fighters become skilled through a brutal Darwinian process once they arrive to fight. The foreign fighters are often more useful in an insurgency, which Ukraine may become. For guerrilla conflicts, the foreigners' dedication is vital, and their more limited firepower is less of a disadvantage.
Nor are the foreigners necessarily good for the very people they seek to defend. One study found that when foreign fighters deploy, violence against civilians goes up. Far-right militias are already declaring they plan to exploit the war. Often the foreigners are more zealous and, because they lack friends and family in the community in contrast to host country nationals, they can commit atrocities without fears of reprisals against their loved ones.
In conclusion:
In general, private war is a bad idea even in cases like Ukraine, when there is a clear victim state and villain state. If governments believe Ukraine needs more support, they should provide it, not put the onus on individual citizens, especially when such fighters can make things worse for the country in question and pose long-term dangers. At the very least, it is vital for home governments to track those fighters who come and go from Ukraine. It is best for governments to encourage those motivated to help to dedicate themselves to fundraising, medical care and other vital but non-violent activity, ideally helping refugees and other vulnerable populations outside the war zones.
https://www.lawfareblog.com/foreign-fig ... -and-risks
