According to a war game organized by The Atlantic with the help of retired air force colonel and specialist in the field Sam Gardiner, which simulated preparations for an assault on Iran by the next American administration, be it Republican or Democrat, such an assault could involve any or all of three separate strategies: (1) a punitive raid on key Revolutionary Guard units to retaliate for Iranian actions in Iraq and elsewhere, (2) a pre-emptive strike on all possible nuclear facilities or (3) the forceful removal of the Mullah regime from Tehran in a regime change operation.
The war game's panel decided that the first two could be carried out independently but that the third would require the success of the first two as preparation. In reality, the second option — a pre-emptive strike against Iran's nuclear facilities — is the one most often discussed.
Any military action by the U.S would likely come as a result of immense pressure from Israel mounted on the most heavily pro-Israel President for decades. Israel is likely pushing for the regime change option. The consequences of such an operation would be a catastrophic conflict liable to engulf the entire region.
The earliest retaliation would probably come in the form of missile attacks on Israel and other U.S. allies within the range of Iranian missiles (1,280 kilometers). Iran might also decide that a bloody defeat for the U.S. is more important than preventing Iraq from becoming a failed state, and begin exerting their significant influence over the majority Shia militias in Iraq to more heavily join the war against U.S. forces. Iran has so far discouraged the Shia communities from becoming involved in the insurgency.
This would mean that the number of U.S. forces in Iraq would drop considerably for the first time, as significant numbers are used in the invasion of Iran. This would coincide with the most dramatic rise of violence against U.S. forces since the Iraq invasion began.
If the Iran invasion did not go according to plan, the subsequently shrinking number of U.S. troops in Iraq could shortly find themselves unable to control the rising violence and be forced into a hasty withdrawal from the Green Zone. Such an outcome would be seen as a defeat and would empower the Jihadis for decades to come.