[agere_contra]

The US is not in a position to sustain another major war

Land wars far from home with complex lines of supply through the Middle East, perhaps not. However the US is certainly in a position to win a large number of simultaneous sea-air wars.

[old republic]

The US's weakness lies mainly in its finances not its military. We definitely have military superiority right now, but we are overstretched and unable to fight a major land war at present. Also, as you noted the US's air and sea superiority is unmatched. However, if a war with Iran starts, then the Iranians will cut off a huge portions of the worlds oil supply. This in tandem with the the oil spill in the Gulf of Mexico, as well as the increased demand of energy any war would cause, would result in a huge spike in oil prices and thus the cost of any such war would be phenomenal to the US.

The US has already spent itself to the breaking point and if it attempts to print money to pay for a huge war (because no one is going to lend it to us), we could have a currency crisis. The US's enemies don't have to beat our air/naval superiority. They only need to bleed us financially to death and that may be enough to overcome the might of the US military.