War Plan Red - The US plan to invade Canada.
While that is interesting in its own right, one should be careful to read too much into the existence of such plans. In the 1800-hundreds more emphasis was put on military planning as opposed to execution in the face of what had already happened. It was thought (largely correctly) that to gain and hold the initiative you had to already have considered possible ways of conducting operations and have ready plans for what it would take to carry those operations out. (The specialised staff officer comes from this period, the general staff being responsible for this analysis and planning).
However, in order to keep the necessary knowledge alive, major engagements being few and far between, a system whereby these staff officers were continuously trained by (more or less) dreaming up new scenarios, analysing them and making the plans to support the operations that that scenario would entail. As for keeping the updated, what do you otherwise do with the new young officers that can't be trusted to do stuff that actually has an effect on anything? You put them to update "War plan Z" or something. When they're finished you get to tell them why and where they got it all wrong, and send them back to their desks which both keeps them busy and out of everybody's hair, and also impresses the important lesson on their young and impressionable minds that they don't actually know anything and should mind their tongue and manners when the grown-ups are talking.
That these plans are kept secret isn't necessarily due to military necessity, but rather to make sure that the equivalent young know-it-all- politician doesn't get wind of them and create all manner of problems getting in the way of the work the grown-ups have to actually get done. It's more of a "conveniently secret", than actually sensitive (as these plans are on such a high level that most of the data they use aren't secret/unknown to begin with. What forces are where, what their general capabilities are and what the map looks like are no great secrets.)
So, if you look through the archives there would and should be plans for almost everything including invasion by space aliens via flying saucer(s) as the plans themselves aren't that important in the greater scheme of things, but the planning process, and keeping that skill alive is very valuable.