In 30-40 years, China, India and Brazil are all predicted to be superpowers with economies surpassing that of the United States, that is a fact. What happens if one of these countries decides to throw there weight around? Lets say, if China decides to invade Taiwan, and we say "no or else". We may just find ourselves in WW3!
Also, the EU (whether we like it or not) is expected to become much more "federalised" and will probably have a single foreign policy and armed forces, its already planning to become a counterweight the US, we all know that Britain contributes more than anyone to the EU force and in the future we will have a big role in it, so we're gonna need a fully-effective force. What i'm trying to say guys, is that the world is changing fast, the day of the one superpower is soon to be over and were gonna find that our enemies may just be a little bit bigger than small autocratic regimes.
Hasn't history taught us anything? 60 years ago we were fighting WW2, 20 years ago the world was on the brink of total nuclear destruction...