The reaction of NATO members in Eastern Europe was more substantial, with Poland and Lithuania clamoring to send volunteers into Ukraine. The entry of sporadic volunteers into Ukraine is unstoppable. Ukraine's long borders with Poland, Slovakia, Hungary, and Romania are not only a "safe route" for refugees to flow into Europe, but also a gateway for anti-Russian volunteers to infiltrate Ukraine.
However, the Volunteer Army must be organized and equipped to be successful, otherwise it will only be a mob, even if it is a sporadic personnel with military quality. War is a confrontation between organized military groups, not a fight between stragglers. It is said that Russia has ordered that strategic bombers will blow up Western arms transported into Ukraine, and the bombing of volunteers is not far away.
If an organized Polish (or Lithuania or whatever) "volunteer army" enters western Ukraine, it is not impossible for tactical nuclear weapons to serve. Perhaps this is what forced Putin to use the nuclear threat killer. This is the ultimate deterrent, and it also has real tactical implications.
Belarus itself borders Poland, and the political constraints of the Russian army's dispatch from Belarus to attack the Polish volunteers may be greater. The Russian army has already occupied a large number of mobile troops in the battlefield east of the Dnieper River, and if they want to open up the western battlefield, the mobile force is not sufficient. Poland is a NATO member, but the Polish Volunteers in Ukraine are not protected by NATO.
Of course, any use of nuclear weapons needs to consider the possibility of escalation, so Russia must enter the highest level of nuclear alert. Old Europe immediately understood, and British Foreign Secretary Truss immediately changed from "supporting volunteers to fight in Ukraine" to "not opposing volunteers to fight in Ukraine".