Sorry for asking, but how is this part relevant to the question? If the invading country were led by someone nice, or if its invasion were not a part of an ongoing revanchist campaign, would the answer to the invasion be any different?
My response as a private person would be no response. In an imaginary scenario where I had any political power, my response would be similar to that of India, China, Mexico, Cuba, Egypt, or United Arab Emirates. It probably makes me a bad person.
Equally I would want the current combatants to take a step back and have a grown up discussion about NATO expansion, security guarantees, ending the bloodshed - you know the boring nitty gritty stuff. I'm not dumb enough to cheer on a nuclear strike, regardless of who launched it.