This one almost belongs in ASB.
Your POD would have to be Al-Qaeda being a Shia group and going from there.
Make the 9/11 attacks from Hamas, and then we're off...
But it still leaves a hostile Iraq up there to give the US forces in Iran a problem. Sure, they hate the Iranians, but they're not above stirring up a little trouble - maybe slipping guns to restive Shia in Basra Province ans hinting "Why don't you go help your cousins out?" - which both removes a problem from Iraq AND causes a problem for the Americans - a double win for Iraq - triple win, if some Iranians get killed in the process.
Really, really, would hate to see the US getting involved in a ground war in Iran. The place sucks.
It really, really sucks. I used to look at the maps and sattelite photos of the place and wargame how we'd do it, and came to the conclusion "Not just no, but HELL no."
Quagmire, just doesn't begin to describe it.
Sure, the Saudis and their buddies would be happy to support us, maybe even send troops, definitely fund a lot of the expense, but...it would suck, bad.
Body bag city and another wall of names bigger than the Viet Nam wall in DC. That's my feeling - and that's the US side.
Freaking armageddon for the Iranians. CBUs by the BuFF load, ARCLIGHT missions running nonstop, fast as we could ship the bombs in. Bodies in windrows for as far as the eye could see...FAR worse than Iraq and Afghanistan put together and doubled or tripled.