Talking more of a balkanisation/USSR style of collapse (where some countries go independent instead). Lets say the USSR collapses under its own weight, then the US collapses in the 90, maybe via poor economic conditions and overextending itself in the middle east or something like that (military spending that is too big, continued borrowing leads to worse economic situation from credit rating etc).
Its hard but how could it possibly happen (or what/who needs to come in to make it happen).