Both US and Russia have tactical nuclear weapons that would allow continued settlement outside of 2-5km (better being upwind), with Russia (Iran for that matter) having Hypersonic delivery. So there is a “no other state” solution. Air burst weapons limit long term radiation in earth. Hiroshima (air bursted) was deemed habitable after the fires shut off. Both Hiroshima (area) and Nagasaki had normal life expectancies for those who moved there after the event. So an “empty the other state” solution exists too.