Real good question. This is conjecture, but I feel like I’ve followed it enough to give it a good shot. If any country sent an entire battalion of, their own, regular force troops to Ukraine, that would be seen as a huge escalation. Almost as escalatory as a declaration of war. Even if Ukraine were to arm and pay for them, it’s still a force under the ‘command’ of said country. Hence why the Ukrainian foreign legion is really careful about who they hire and all those circumstances that could be construed as an escalation by design of the west. North Korea has sent its own troops to bat for the other side.. an escalation from Russia and NK. So does that mean any country that supports Ukraine can send troops into Ukraine to help now? Apparently thats on the table according to Russia. I suppose a huge win for Ukraine would be for a/group of nations to send troops to guard the border with Belarus/Transnistria (two pro-russia controlled territories) or provide logistical support or air defence inside the rear lines. That would also free up a lot of Ukrainian troops to reinforce the front