Great piece by Edward Luttwak. I think he is on to something.
Just as the Sunni threat to majority rule in Iraq is forcing Sciri to cooperate with the U.S., the prospect of a Shiite-dominated Iraq is forcing Sunni Arab states, especially Saudi Arabia, the Emirates and Jordan, to seek American help against the rising power of the Shiites. Some Sunnis viewed Iran with suspicion even when it was still under the conservative rule of the shah, in part because its very existence as the only Shiite state could inspire unrest among the oppressed Shiite populations of Arabia. More recently, the nearby Sunni Arab states have been increasingly worried by the military alliance between Iran, Syria and the Hezbollah of Lebanon. But now that a Shiite-ruled Iraq could add territorial contiguity to the alliance, forming a "Shiite crescent" extending all the way from Pakistan to the Mediterranean, it is not only the Sunnis of nearby Arabia that feel very seriously threatened. The entire order of Muslim orthodoxy is challenged by the expansion of heterodox Shiite rule.