I keep thinking, what happens if these countries just say fuck it...Russia you can have the frozen conflict zones. We agree...new borders, also lol the now undisputed country that is overwhelmingly fearful of you and anti Russian government is in the EU and NATO. It is the ongoing conflicts that are part of Russias strategy in preventing memberships. What if they just give them up?
It's an interesting question. I would guess they have to sponsor visas for those Russian fighters.
The US government identifies visas in health care that are difficult for us to do and as such we refuse to sponsor Iranians in some places I've worked. An American or resident of Iranian ethnicity would not be the same.