War using what?
The US doesn't exactly have any military forces to spare for another invasion. Pull them from Iraq and the place will melt down faster than an icecube in a supernova. Pull them from Afghanistan and, besides the fact that there aren't enough troops there to hurt Iran if they were used, everyone else engaged there will use it as an excuse to get the hell out. Use any troops that remain stateside and you doom both Afganistan and Iraq as there will be noone to replace the troops already there.
Of course, they could bomb and launch a bunch of cruise missiles without too much trouble, but actual war? Not unless they intend to start drafting people to swell the ranks. And considering how easy it would be for Iran to return the favour given the number of american troops in Iraq, it would be a very bad idea to start bombing unless prepared for real fighting as well.