Actually, if there were a Civil War, I highly doubt that Russia and China would bother (with some caveats).
Russia is already concerned with it's border with China. Another Civil War in the US would see the pullback of US military assets from around the globe. Nature abhors a vacuum, and in the Far East, Japan, the Philipines, Vietnam, India and possibly Australia would eventually see serious turmoil with a China that flexes its muscles.
Europe would have much to worry about with a Russian bear who decides it's springtime and decides to go foraging on its western border.
The Middle East? Well, there's not too much to say there. They'll go about happily blowing each other up like they have for thousands of years.
The caveats:
A protracted civil war in the US would eventually grow perceptions of a weakened country, and THEN you would possibly see certain parties interested in taking a bite of the apple. I do think the "takers", as it were, would be the ones to allow a foreign force into the US at that point, because in a protracted civil war the power base of the takers (cities) would be decimated.
Then there is desperation and all those pesky nukes....
A protracted civil war in the US isn't possible. The cities' food and water supplies would be denied.