Usually the Cherokees or the Iroquois (more often its the Cherokees) attack and capture one of The 13 Colonies and then lose it to rebellion and that region then becomes The United States. Every time I have played GB the new U.S. quickly attacks either the remaining colonies or GB (if as GB you capture, say, Georgia from the Cherokees). The game seems to be coded to do that since in my experience this happens all the time both before and after the last patch.