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First of all, if Israel bombs Iran, the USA will be assumed automatically complicit, and probably will be. hardly a matter of Iran attacking the USA when its the other way around.
Then we can ask how the mightiest military in the world did in Vietnam. Or for that matter, is doing in Iraq and Afghanistan. The USA may be due to pull out Iraq, maybe mission accomplished at the end of 2011, but if Iran gets attacked, chances are, a Shia majority Iraq will side with Iran. As for the US mission in Afghanistan, both Pakistan and Afghanistan would almost certainly be in open rebellion as the USA would almost certainly lose its supply route into Afghanistan. Not to mention the fact that Egypt, Turkey, and even Saudi Arabia might side with Iran.
Its a little hard to be bullish on the mightiest army in the world when just Iraq and Afghanistan almost exceeded its capacity for a decade.
I would also consider the possibility that Israel attacking Iran as a remote possibility, if not
for the link I submitted and the fact there are now strong indications that the UK may join the Israelis.