Wow! He really doesn't know? He really has no clue as to why America doesn't have anywhere near enough soldiers to do the job in Iraq? That Donald Rumsfeld decided long before the Iraq War that it would be fought on the cheap, with minimal troops? Rumsfeld is now trying to dodge the blame for his underestimation of what was needed, but he was following a pretty clear philosophy:
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Throughout the period, Rumsfeld continued to push for increased outsourcing, especially in basic administrative services such as information technology, security, and maintenance. "Why is the Defense Department one of the last organizations that still cuts its own checks?" he asked in 2001 in a harbinger of contracting-out to come. "When an entire industry exists just to run warehouses efficiently, why do we still own and operate so many of our own? At bases around the world, why do we pick up our own garbage and mop our own floors rather than contracting those services out, as many businesses do?"
Problems with this business-like, efficiency-based philosophy surfaced quickly. Cooking meals for troops in the field was outsourced to private contractors:
The essential problem of course, is that civilian contractors cannot reasonably be expected to serve under enemy fire. Outsourcing meal preparation to private contractors meant that front-line troops couldn't get hot meals. Privatization caused still further problems:
According to the Newhouse News Service, "U.S. troops in Iraq suffered through months of unnecessarily poor living conditions because some civilian contractors hired by the Army for logistics support failed to show up." Not surprisingly, civilian contractors  and their insurance companies  get spooked by war zones. The Financial Times reports that the dismal performance of contractors in Iraq has raised strong concerns about what would happen in a war against a serious opponent, like North Korea.
Rumsfeld also felt that fast, light forces could do the job. Well, they could have, if the job had been limited simply to demolishing Saddam Hussein's army and if fighting a guerrilla war had not been necessary. As it was, even something as early as guarding the al Qa-qaa ammo dump was beyond the capacity of Rumsfeld's "fast, light" forces.
As the Chief of the Iraq Survey Group, Charlie Duelfer pointed out, the US didn't have enough troops to both seize Baghdad and secure weapons sites.
Rumsfeld's two ideas, "fast, light" forces and privatization failed when applied to real-world conditions. Rumsfeld is very tired of hearing about Eric Shinseki, but the fact remains, Shinseki was right and Rumsfeld was wrong. The US needed several hundred thousand troops to secure Iraq once the main-force fighting was over. Of course, a draft would have been necessary to have built up a force of the size needed and Bush has never called for any real sacrifice to fight his little vanity war, meaning it was doomed from the start.
Unfortunately for David Brooks, there's no mystery as to why US forces in Iraq are understrength. It's the result of Donald Rumsfeld's ideas concerning efficiency and privatization.
FURTHER THOUGHTS: In a Quick & Dirty Guide To War (At least I think it was in this book, I know I have the author right), James F Dunnigan points out that there were two methods of replacing casualties in World War II, the American way and the German way. The Americans borrowed from their method of re-supplying grocery stores and automobile dealerships. The officer in charge of the unit would count up how many "units" (men) were lost to combat and order up the appropriate number of replacements "Send me 17 riflemen, 5 tank-crew members and an artillery officer."
The Germans, drawing upon centuries of land warfare, kept units in the field until they were worn-out and needed a break, sent them to the rear, new personnel would then report, unit-wide training would commence and the veterans trained the newbies until that unit became a solid, cohesive group. Until it had become a unit where everybody knew everybody else and they had all gained confidence in each other. When the German unit reported back to the front, casualtes among the newbies were proportionately not much greater than they were among veterans.
By contrast, when American newbies reported to the front, casualties among them were proportionately much, much higher than they were among veterans. American newbies reported without knowing anybody, without having had any experience working alongside the veterans, without having gained any confidence in their new partners. No two ways about it, the German way was far superior.
Using Rumsfeld's corporate-influenced theories of warfare, the American Army tries to send cooks into the field that aren't even soldiers, that don't have any training at all. Not surprisingly, it's like tossing people into a furnace. They're almost certain to get burned up.