The U.S. does not take a strict interpretation of the 10th ammendment. If it says nothing about something they will think they have the right to do it.
I know, the founding father's were not infallible gods. They couldn't see the future, and since in the 1700's air planes and spaceships were not real things. They did not talk about it.
You are right about the Army. But Air Force and Space force are not covered by the Constitution so there is no limit for them being indefinite like the Navy.