This is a very Hollywood action film way of seeing the world. In the case of an American civil war, it's unlikely that it will be fought using nuclear weapons, and in that case, it's unlikely that they would use any on say, Chile, or Australia.
Europeans are screwed though.
If you live in Chile, the main danger is not that the U.S. drops a nuke on you. It's that Pakistan (freed from fear of international condemnation) drops a nuke in India, which then can no longer export rice to Saudi Arabia, where revolt breaks out, which cuts off the flow of oil, which makes Chile's economy grind to a halt.
So it’s not the Hollywood action movie view of the world. It’s just the standard 2005 version of Pax Americana make-believe.
Meanwhile Israel has attacked Iran and its US ally said “we want to do that too”.
India is also a nuclear state, so this is pretty unlikely unless the wheels really come off the deterrence strategy. North Korea attacking South Korea perhaps, but even that seems unlikely as it would greatly anger NK's closest ally of China.