When people say "the US" they usually mean the American bourgeoisie. They fight smaller imperialists when their interests clash, but their biggest enemy will always be the international working class.

As for the American working class, their biggest enemy is the one closest to home, the American bourgeoisie. The American capitalists directly exploit them in the work place, but are also crushing the workers all over the world and devouring the Earth in search of ever greater profits. They need to be removed from power and cease to exist as a class if humanity is to keep existing at all.