The Anbar campaign we have been fighting for the past year that Bill Roggio detailed quite well has damaged the insurgency badly. But, you are right that we didn't have enough US troops to seed Anbar as we have been able to slowly do over the past year only because we have been getting more Iraqi troops online.
So, I don't think the big mistake was the number of US troops, I believe the fact they planned on a very tiny 60,000 man strong Iraqi Army for the first year of the war was the big mistake.
The problem is that if we had sent more troops, or kept the Iraqi army intact, the insurgency would not exist in its present form. Our failure to get the basic infrastructure of Iraq up and running, and our limp wristed responses to the early advances of the insurgency combined into making their causes a popular one. We've finally gotten our act together, and are grinding the insurgency down, but in all fairness we stood by and let a number of little fires burn into bonfires. We did a lot of things very, very well, but there were a few that we could have done differently. I hope that the next time America finds herself considering a foreign occupation, people benefit from our example.