Showing Off My Ignorance, #48213

I have a question about the sturcture of our armed forces. According to this we have just over 1.4 million active duty soldiers, marines, sailors and airmen. As of September this year we have approximately 170,000 people serving in Operation Iraqi Freedom (137,000 as ground forces), a considerable portion being composed of Reserve and National Guard units. Some of these units, I understand, are now preparing or are serving their second tour in Iraq. My question is this: why is what appears to be a significant dependence on our reserve forces necessary to field a 170,000 person expeditionary force in Iraq (a deployment of about 12% of our active duty force)? Is our active duty military so short of support units that the Reserves are no longer reserves?

My suspicion is that our force structure (is that the right phrase), may be left over from a time where economy was more important that staying power. I freely admit my ignorance however, and am willing to hear better explanations.