More than enough people have volunteered to defend our country. The problem is that they are being used to pacify and babysit other countries. 140 other countries, to be precise.
And to add to the problem, there are many people, such as myself, who don't join precisely because of this chronic misuse of the military.
Bring our troops home from Europe, Bosnia, Serbia, etc... , treat them with decency and respect, pay them a fair wage, and you'll find that when the US needs defense, (not play World's Policeman), we will have plenty of manpower.