i remember when the demilitarized us and the coalition had to fight in korea, where they mobilised in just a few days to busan (the us presence when north korea attacked was less than 300 advisors), then drove them all the way to the chinese border before the chinese stepped in with masses of soldiers which still led to a stalemate. that's what the demilitarised west did and that's with china being war experienced. and since you've talked about nk alone and not china... the war would be much more one-sided, nk troops would reach nowhere near busan.
Serbia