My fear (from inside the US) is civil war if Trump loses the election. He and his minions are working overtime casting doubt on the outcome of the election before it even occurs. If he loses, his supporters, who are heavily armed, will refuse to believe the election was fair and take up those arms to keep him in office.
I don’t see nuclear war as a possibility. Our military has made it clear that they will not follow his orders if they are illegal. Although they are supposed to be politically neutral, all of the military commanders are united in their opposition to Trump.