we've got a country thats split with a good majority of the populace pro-western or maybe not pro-western but progressive and opposing to the anti-western bullshit so I don't see them forming any kind of resistance to an early US occupation under the current administration as long as we don't do anything too stupid. And then we've got Iran's administration and Iranian security forces. I think if the US actually decided to go to war with Iran and the progressives were pissed off enough at Ahmadinejad it would be over pretty quick.
Yeah this is what the Americans thought when they invaded Canada! Oh they'll just surrender they like us, right?
The fact is that when one country marches into another one the country being marched into immediately galvanizes against the invader. Generally speaking, the population that supports the invader generally doesn't do dick all except cheer and wave miniature American flags, whereas on the other side you have an equal number of people shooting at you. With guns. And rockets.
People thought this about Burma, too. Everyone hates the junta there, but if the US (or any other nation) invaded they'd all leap up to defend whatever bullshit political leader they have because he's
their bullshit political leader and even if they plan on ousting him first chance they get, at least they'll do it on their own terms and not thanks to some foreigners. Nationalism pretty much dictates that unless one foreign power controls your government you don't want to see any other foreign governments meddling directly, and the best way to spark nationalism is to invade a country.
To summarize: DO NOT INVADE COUNTRIES THEY NEVER WANT YOU TO