But even airstrikes would clearly be considred by Iran, and probably most of the world, as a declaration of war. The US can't handle a ground-war with Iran, regardless of whether the US invades Iran or if Iran decides to protect itself and goes after the US forces conveniently stations across the border. You don't believe that Iran is just going to sit there and take it, do you? In the case of bombing, they would have world opinion squarely on their side (or rather not on their side, but opposed to their enemy which is pretty much the same thing), not to mention Russia and China having a significant stake in contidued stability in Iran, as well as their own AA defences which are probably significant (dunno about that, pulling stuff out of my ass, but its not unlikely.)