If you read this Washington Post account of fighting in southern Iraq a couple of things become clear. One is that the United States is deeply involved:
U.S. forces in armored vehicles battled Mahdi Army fighters Thursday in Sadr City, the vast Shiite stronghold in eastern Baghdad, as an offensive to quell party-backed militias entered its third day. Iraqi army and police units appeared to be largely holding to the outskirts of the area as American troops took the lead in the fighting.
The other is that nobody in U.S. policymaking circles really thinks we have a dog in this fight:
The U.S. officials, who were not authorized to speak on the record, said that they believe Iran has provided assistance in the past to all three groups -- the Mahdi Army; the Badr Organization of the Islamic Supreme Council of Iraq, Iraq's largest Shiite party; and forces loyal to the Fadhila Party, which holds the Basra governor's seat. But the officials see the current conflict as a purely internal Iraqi dispute.
Some officials have concluded that Maliki himself is firing "the first salvo in upcoming elections," the administration official said.
Basically, we have our troops, risking their lives and killing people and all for the sake of helping some Iranian-backed militia groups fight some other Iranian-backed militia groups and, yes, the groups we're supporting initiated this battle without clearing it with us. But we need to back them, because George W. Bush has staked his precious credibility on his alliance with Nouri al-Maliki, so if Maliki says American blood and treasure will be expended fighting the Mahdi Army, then so it shall be expended.
Alternatively we could just leave and let this people sort out their own problems.