The thing about the draft in the USA is that waiting until the bullets begin to fly is not a good idea. It will take time once the "go" button is first before the first group arrives at their training center. If things go without a war, they can be discharged, if not well they are needed. If it comes to nukes, even soldiers who have just finished basic (or almost there) can be used for the many purposes they would be needed for. In a post nuclear attack America, the government can't have enough trained and disciplined forces to try and hold things together.