Victor Davis Hanson considers the question, and says it is not a matter of numbers, but choice of strategy and tactics:
"Our problem, in short, is not that Donald Rumsfeld is fighting wars with too few troops and then being too stingy in allotting occupation forces. Rather, our concern with restraining the use of the vast power of those already in the field has put us at risk of creating self-fulfilling prophecies. We have seen a Gulf War I and now a Gulf War II. Gulf War III is surely on the horizon if, failing to learn the lessons of the last two victories, we once more remove the stakes from the hearts of seemingly defeated and moribund killers."