A little off-topic, I know, but there have been other contingency plans to go to war with Britain. I'm afraid I can't summon up a source for this, but I recall reading once we had a plan for war with the UK as late as 1945-50. The rationale was that Britain would emerge from WWII damaged, broke, and without an empire and the US would be the dominant world power, Britain might be tempted to do something to re-establish herself as a world power. This is not quite that far-fetched, consider what happened in Suez just a few years later. England and France tried to throw their weight around one last time, and we forced them to back down. The UK got over it, but I don't think the French ever did.

It's the military's job to do this sort of thing, so that if you do have to go to war unexpectedly you don't get bogged down or caught with your pants down.