1. US starts on Iran, the religous factions unite and a war starts, USA are mostly religous Christian's vs Middle East's Islam. Other religons see this as a chance to exterminate the Islamists and join in, then turns into holy war with the USA, the UK is shortly forced to join making it very difficult for Europe indeed.
2. You got a war, USA vs Iran. then stupid or not Iran attacks US supplied Isreal with a nuke, the Jordanese, the Palistinian's, the Eygptian's, the Turk's and the Greek's all respond with a mass ground attack on Iran. The Pakistani's join in because they could be next in the nuking process. The Russian's launch a massive air strike against Iran. That then prompts the Koreans to launch a nuke over China to the USA while the USA is down, the Chinese and the Japanese immediatly respond by begining a mass force invasion of Korea because of ignorging what the Chinese and Japanese said about nukes.
3. You get propaganda wars on Iran and in the end, Iran starts on the USA with 5 nukes and wipes the USA out completly. then you got every other country that has nukes and wipes the Iranian's off the face of the earth with over 40 nukes.
Its a possiblity no matter how stupid. I hope the USA is fully prepared for what could happen, as an example of 3 possiblities above.
as a pointer on iraq, the USA could'nt of kept iraq anyway, the insurgents have made it clear since day one that the USA is not welcome, so far thousands of dead Americans and hundreds of dead innocent iraqi's, do you think that will stop?.
Besides that the fact remains is the USA wants a ally in the Middle East, aside Isreal, to help them point out what goes on that continent, witch means the USA have a problem. They either; stop investing in Iraq and bomb it sky high or continue the safeguard process...