The Saudis are making it clear that if the US leaves Iraq, they will step in:
Saudi Arabia's ambassador to the United States, Prince Turki al-Faisal ... said in a speech last month that "since America came into Iraq uninvited, it should not leave Iraq uninvited." If it does, one of the first consequences will be massive Saudi intervention to stop Iranian-backed Shiite militias from butchering Iraqi Sunnis.
...Options now include providing Sunni military leaders (primarily ex-Baathist members of the former Iraqi officer corps, who make up the backbone of the insurgency) with the same types of assistance -- funding, arms and logistical support -- that Iran has been giving to Shiite armed groups for years.
Another possibility includes the establishment of new Sunni brigades to combat the Iranian-backed militias. Finally, Abdullah may decide to strangle Iranian funding of the militias through oil policy. If Saudi Arabia boosted production and cut the price of oil in half, the kingdom could still finance its current spending. But it would be devastating to Iran, which is facing economic difficulties even with today's high prices. The result would be to limit Tehran's ability to continue funneling hundreds of millions each year to Shiite militias in Iraq and elsewhere.
Both the Sunni insurgents and the Shiite death squads are to blame for the current bloodshed in Iraq. But while both sides share responsibility, Iraqi Shiites don't run the risk of being exterminated in a civil war, which the Sunnis clearly do. Since approximately 65 percent of Iraq's population is Shiite, the Sunni Arabs, who make up a mere 15 to 20 percent, would have a hard time surviving any full-blown ethnic cleansing campaign.
In this case, remaining on the sidelines would be unacceptable to Saudi Arabia. To turn a blind eye to the massacre of Iraqi Sunnis would be to abandon the principles upon which the kingdom was founded. It would undermine Saudi Arabia's credibility in the Sunni world and would be a capitulation to Iran's militarist actions in the region.
To be sure, Saudi engagement in Iraq carries great risks -- it could spark a regional war. So be it: The consequences of inaction are far worse.
Policy options in Iraq just seem to be getting worse and worse...