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You said: Obviously the US military would roll over Iraq, and it's just as obvious it would roll over Iran, though it would be a bit harder...
This is an understatement. Iran's population is almost three times that of iraq. The Iranian army has a small air force, but numerous army bases, a well trained army, and a land mass 4 times that of iraq. While most Iranians are not fond or the current regime of mullahs, they would fight American invasion tooth and nail. They are a proud people who perceive their country as a once great nation that will be great again. Most are confident in their right to develop nuclear weapons for security reasons.
Likening an invasion of iraq with an invasion of iran is incredibly misleading. For an excellent study of the implications, read this report by Oxford Research Group:
http://www.iranbodycount.org/
The report notes that an American attack against iran would undoubtedly have a unifying effect on the many different political and social factions which now exist within Iran, accompanied by a lifting of suport for and morale within the Royal Guard, and a commensurate boost to recruiting.
The first reaction to American airstrikes on iranian nuclear sites would likely be a furtive re-building of the nuclear capabilities, coupled with a witdrawal from the non-proliferation treaty (of which Iran is currently an abiding signatory). We would likely see increased Hezbollah activities in Lebanon, sabotage of west gulf oil facilities, and a world-wide reaction by muslims that would ultimately lead to more suicide bombings, more recruits for Al Quaida, and another generation of young muslims (this time iranian) that are willing to die to strike at American hegemony.