By not expanding the armed forces to fight this new/old enemy. Instead numbers are pretty much unchanged since 2000, with a little shifting from Navy and Air Force to Army. Marines have not been expanded. There has also been some shifting of uniformed manpower from tooth to tail, with the tail part being taken over by contractors. There has been significant use of reserve and guard forces as well, although mostly for combat support roles. (Of course in this type of war, everyone is "on the front line", even if they only, in fact especially if, they only drive a truck). Thus budgets have been increased, but other than the call up of reservists (and call back of recently released "regulars") the forces available have not increased. There are more threats that need addressing, or coverage, but the forces are stretched just with the Iraq and Afghanistan operations.
Normally during wartime, the military gets all it needs to do it's job. This time we are robbing Peter to pay Paul. Some Army posts are not able to pay their utility bills, and maintenance and upkeep has gone undone.
You do make some good points. Has the pentagon asked for an increase in personal? It seems to me these are the people who know more about just what is the right size of forces we need.