Rule Britannia is pretty gung-ho all right, a bit of a technicality to say it is not actually about war. It is couched in fiarly self-defense sort of turns, but the arly part of the song is basicaaly saying that no tyrants will ever succede in conquering Britannia,`because the navy will get the bastards before they get here. Which is a reasonable distillation of a thousand years of foreign policy, driven by the embarrassment of William the Conqueror's easy ride.
   BUt a little later in the song we can find the lines:
"All thine shall be the subject main
And every shore it circles thine".
Now, the "thine" here is Britannia, and "the main" is the sea.. There is no conceivable interpretation of this sentiment except the obvious: the author wishes Britain to conquer every bit of the world accessible from the sea(which, let's face it, is pretty much what happened in the century following the wirting of that song). I think it's a great song. But I have to add that that doesnt mean I literally think we should go out and conquer all the fuzzy-wuzzies again.