that's the one that would hurt the most...
Tactically, what is important for US forces in the ME is first and foremost, a layered ballistic missile defense. Its primary defenses are directed at first protecting our fleets, second, protecting our bases in Iraq and Afghanistan, third, protecting Israel in exchange for their not retaliating with nuclear weapons, and fourth, to protect the Iraqi and Saudi oilfields.
Our ground forces are anticipating a Tet-style offensive in which several thousand Iranian agents organized into networks attempt to destabilize southern Iraq enough so that an Iranian army may invade, thus keeping the three US divisions tied down in fighting there and unavailable for invading Iran.
We are also very aware of Syria, and how Iran intends for them to be a second front against US and Israeli forces. For this reason, the rapid conquest of Damascus may be already part of our battle plan. This would essentially turn Syria over to their 80% or so Sunni majority, taking power away from the Iran and Hezbollah-allied Alawite Shiite minority that rules Syria.
A ground invasion of Iran directed towards Persia is unlikely. However, that may not be necessary, as once the Iranian nuclear infrastructure, military and Revolutionary Guard has been reduced, they would be unable to reconstitute their nuclear program if deprived of the resources to do so.
This would be done by partitioning three or four minority regions of Iran, and giving them over for protection to their respective adjacent nations. Iranian Kurdistan would join with Iraqi Kurdistan; Iranian Arab Khuzestan, with most of Iran's oil, would be joined with Arab Iraq; and mineral rich Iranian Baluchistan would be joined with Pakistani Baluchistan. And possibly even the Iranian Azeri territories joined to Azerbaijan.
This would avoid the need to invade Persia, which would be a most difficult effort, yet would prevent the Persians from rebuilding their nuclear program. And because the partitioned territories are closely related, they would be strongly defended by their adoptive nations.
I am not convinced though, that Iran will merge with it's like nation neighbors - although it sounds like given a chance, it would work.
that's the one that would hurt the most...