The limits of the modern military: occupation is no longer an option?

Since the beginning of the Iraq war, I've felt that three things were under-analyzed:

1. The impact of Turkey's withdrawal on the original war plan. Few now remember that the original plan assumed a massive military contribution from Turkey.
   
   
2. The likelihood that Rumsfeld always assumed Iraq would be partitioned into 3 parts (With US clients keeping much of the oil of course. I suspect Bush was not informed of Rumsfeld's opinion, though Cheney would have been.)
   
   
3. The reality that the US never had a large enough military to occupy (vs. invade) Iraq and that the military knew this from the start.

Phil Carter has written quite a bit about the limitations of the volunteer army, now an article in the June Atlantic provides some more detail on the training and recruitment challenge. I was struck by this number (emphases mine)

The Army We Have

... In the prime age group for recruitment (17 to 24 years old), 7 in 10 are ineligible for military service, Army officials say. More than half the members of this youth cohort are disqualified for moral, mental, or medical reasons: They have had too many run-ins with the law, or they have gang-related or extremist tattoos; they have had psychiatric treatment for severe mental problems or antisocial behavior; or they have been diagnosed with one or more of a staggering list of medical conditions, from heart murmurs to obesity. Other potential recruits have too many dependents, scored too low on the Army aptitude test, or lack high-school or general-equivalency diplomas...

It's a telling number, though on reflection one can see the sense of it. Obesity alone would eliminate many today, and an effective requirement of an IQ above 100 would eliminate about 40% of the population.

Modern warfare demands a lot of human "capital".

I think one historic lesson of the invasion of Iraq may be that a modern military power cannot occupy a nation. In other words, military occupation is not an option -- for anyone.

Has that been much discussed? It seems rather a significant change ...