If we don't maintain a military presence in Iraq, it is perfectly plain what will happen: Iran will instantly gain control of the southern Iraq oil fields. Iraq doesn't have an army anymore. It is incapable of preventing Iran from acquiring control of its territory. From that vantage, Iran would also effectively threaten the sovereign existence of Kuwait. Then there is the question of how much instability Iran could generate next door in the Shia-dominated Persian Gulf shoreline region of Saudi Arabia, where most of that nation's oil lies. (Meanwhile, there will be plenty more Iran-inspired mayhem in Lebanon and the Palestinian territories.)
And for glorious specifics on how that might happen, check out this [link|http://kunstler.com/Grunt_Baghdad%20letter.html|letter from Baghdad]:
This place could have streets paved in gold...and they know it. But they won't put down their self centered bickering long enough to let it happen. And then there's the "Frankenstein Factor" which has turned loose god knows how many guys on the landscape who are simply whacked out on the violence and listen to no leader. I've taken to describing it as being hooked on "War Crack." Of course certain facets of the cultural lore make for ready made enablers and keep the cycle going indefinitely.Cheerio.
Then there's the fuel shortages, gas lines, rival gangs taking over the few working petrol stations, black market fuelers selling watered down gas from milk jugs at 5 times the going price, busted water mains that don't get fixed, troops, checkpoints and imposter police everywhere. Hatfield vs. McCoy vendettas in full swing with no end in sight, secret hit squads from Iran running guns, money and training to everyone, neighborhood militias cleansing door to door, power lines torn down and looted for the copper, pipelines torched daily, snipers stalking the rooftops who apparently just like to shoot people at random anymore and anyone with a brain and some money beating feet just as fast as they can...if they can.