All-volunteer
There are other people on the board who are much more qualified to speak on this than I am, but I was under the impression when I was considering going into the Air Force, that in the case of national need, my "term of enlistment" was flexible, to say the least. I don't remember the details any more, but they made no big secret of this. The Vietnam War was still on.
The bottom line is that once you sign up, the "voluntary" part of the deal is pretty much over, and everybody knows this ahead of time.