You can kinda see in the screenshots of that post, after you pick a nation, it's constantly you vs. the other nation in the war (high level) maps.
Last night my guild was killing the hardest boss in game (a feat in itself) when the opposing nation came in all the sudden and tried to take us out, and it turned into this epic battle, where we're all trying to kill each other and the big boss at the same time.
Battles like this can turn from 2v2 to 50vs50 when backup is called for.