100,000 extra troops in Iraq would have prevented the massive looting and crime wave that occured immediately after the Iraqi army surrendered. Duh!

The sad fact of the matter is that we didn't have an extra 100,000 troops to send. Troops aren't interchangeable - they have military specialties like logistics, anti-aircraft defense, and over 100 others. You can't effectively just put them in place of combat troops and expect much good to come from it. This is less true for the Marines than the Army. The Marines, bless them, still train everyone to first be a rifleman.

There's also the issue of troops being spread rather thin around the world. Where would you draw down to provide those extra 100,000 troops? What would you do with it was time to rotate them out of the country - somehow scrape up enough additional replacements or do a massive drawdown?

I'll have to see if I can find the current numbers, but I do remember one pertinent statistic from the Vietnam war: at the height of the fighting, the US had roughly 500,000 military personnel in Vietnam. At no time did the actual number of combat troops exceed 100,000. This is known at the "tooth to tail ratio", comparing the number of combat troops to support personnel. The ratio may be a bit better today, but probably not by much.