mmm i dunno hopefully never but i am sure it will happen. but i think that instead of nations individually fighting others it will be the massive trading and economic blocs like the European Union, who will fighting. cause at this stage every continent, with the exception of Antartica has trading blocs and organisations. so i think that more like the EU will develop creating superstates and then they will fight each other. resulting in the domination fo one bloc, creating the world super state.
ok i know its far fetched, but the most likely start of it will be a rouge nation like North Korea deciding they will go South, and if they went South we will know about it!