A massive standing army needs something to do. The "Founding Fathers" saw how badly this affected Great Britain and what a drain it was so the idea was to have a very small army but a ready militia to call up which was the entire point of the second amendment, not just guns for the hell of it. This held sway until the 20th Century when a few more regulars were needed and the tecnical advances forced some more training and hence the build up and need for the Reserve and the National Guard. Then, even through Korea, after the war the military shrunk to a very small scale.
Then along came VietNam..............an entirely different kettle of fish.