Iran will just attack Iraq and the U.S death toll will jump into the thousands.
Iran has a military power of 500,000 people. However in Iran just like in Israel every male at the age of 18 has to serve in the military for 2 years. Doesn't help that 60% of the population are young. They can call up the millions of people they need to fight in any war, but I don't get how they are going to make them fight if they do not want to fight. I can see them wanting to attack Israel for attacking them as a legit reason. As far as the U.S they will put their guns down, but the extremist which number in the 150,000 will probably head to Iraq to fight.
Iran's military is split. One-side is pro regime the other is pro-president. However, these 2 forces have never clashed. The president of Iran wants a more democratic government, but he does not have any power at all. Final say is the Ayatollas.
Also if Israel attacks Iran I see Iran sending in tons of weapons to Hezbollah to do the attacking for them. This time they'll send them whatever they have and Iran has a lot.