The U.S. has a third option.
Tell the Russians to go to hell.
Move 75% of U.S. forces to the Kurdish region of Iraq, bring 25% home.
Watch as the Iranians move in to Basra and head to Baghdad, while the Saudis and Gulf states start to build (hire) actual armies and very quickly are ramping up support for Al Queda and Sunni militias in Iraq as well. Let Iran get “bogged down” with a Saudi-Iran confrontation building.
Meanwhile, the Kurdish area will become the refuge for peaceful Sunnis and Shias fleeing the carnage (which we will not intervene in below the border of the Kurdish area).
At the appropriate time, rested and strengthened, we will not take Iran from the south, but from the east, as the Kurds keep their southern border secure.
Excellent plan.