When someone asks me why the U.S. Army should stay in Iraq I can think of only one major reason. The Roman Empire's desertion of England.
Sometime between 407 and 411 AD the Roman Senate voted to pull their army out of England. It seems that Hadrian's Wall and all the former might of the Roman Army was insufficient to maintain order in that distant colony.
Insurgent Englishmen were killing members of the Roman Legion whenever they ventured out of their forts. The English would pull boards out of key bridges so that Roman chariots would crash into the streams below. Archers hid in the trees and shot arrows at diplomats and soldiers in transit. Buildings occupied by Romans were set on fire. So the Romans decided to leave England.
After the Roman army left, it is estimated that all hell broke loose. Sectarian violence broke out all over the vast island. Records from the post-Roman period are sketchy, of course, but archaeologists believe that the post-Roman occupation violence went on for decades. Far more people died during that period than during the entire period of Roman occupation. Many of the English probably pined for the relative calm they once enjoyed while the Legions occupied their land.
So the United States is stuck in Iraq, probably for decades. U.S. taxes will go to fund the roads, police, sewers, and corruption in Iraq for years to come. No matter who the voters elect as president in 2008, Bush's Colonial War for oil will be with us for a generation.
History seems bound and determined not to be repeated in just this one case. Maybe an Iraqi citizen or two should be taught the lessons of the Roman withdrawal from England. It could save a few thousand Iraqi and U.S. lives over the next few years.