England vs Wales, and it will go as follows:
Scotland will convince the English to invade Wales, for reasons that I haven't been able to make up yet, and the Irish will come over and help the Welsh, again don't know why, then the Americans will fight the British, since most of them think they're Irish anyway.
After this all the Commonwealth countries, such as Australia, Canada, and I can't think of any more, will fight against the Americans, Welsh and Irish, and eventually causing a stalemate.
After some months of stalemate, the many weakened countries will sign a treaty, penned by the Scottish, that has a secret clause in it, which will sign over all power from the waring nations to Scotland. Thus revealing the true plan behind Scotland convincing the England to invade Wales. It's because of many years of secret, and very precise, planning to create...the SCOTTISH EMPIRE!!!
Also, somehow most of Europe, Northern Africa, Turkmenistan and China get involved in this war too, but I haven't thought up how... yet.