Sorry to see you chose to make this thread political, laying blame.
Frankly, I think it has little to do with it. If all of those troops were still in Afhganistan instead of Iraq I fear the resolve of the people, and the US and the world would still be waning. It's difficult because for many it only seems to last as long as the emotion and the adrenalin.
Facts can generally only be discerned by history, and even then they are only interpretted. Look back at WW II, and what was viewed as America's late entrance into that battle. The U.S. wanted proof that what was happening "over there" was a threat to them. Sadly it came.
Terrorism wan't born with Bin Laden. There were plenty of incidents of the growth of militant jihadists before 9-11. They went without much, if any, response, and the movement continued to grow.