Both will have 10 years Prep Time...
The World will Unite as One...
America will be isolated for 10 years, no trades of anything from the outside.
American bases outside America will shutdown or be absorbed by the country they are on.
US Citizens will return to America, vice-versa foreigners will leave the US to return to their countries.
No Spying on each other, maybe after the 10 years Prep
Round 1: America attempts to conquer the World
Round 2: The World invades/destroys America