The top U.S. commander has shifted the focus from Al Qaeda to Iranian-backed "special groups" as the main threat to a democratic Iraq — a significant change that reflects both the complexity of the war and its changing nature.

The shift was articulated this week in Washington by Gen. David Petraeus, who told Congress that "unchecked, the special groups pose the greatest long-term threat to the viability of a democratic Iraq."

Before, American commanders have called Al Qaeda the greatest threat.

There is little doubt that Shiite extremists fighting U.S. and Iraqi forces have received Iranian weapons, although Iran's government denies supplying them.

But Petraeus' comments obscure the fact that the United States has waded into a monumental power struggle within the majority Shiite community — and crucially, that both sides in that struggle, not just the "special groups," maintain close ties to Iran.

The power struggle is only the latest stage in a decades-long competition between the families of the current top Shiite players: anti-American cleric Muqtada al-Sadr and Abdul-Aziz al-Hakim, whose political party in Iraq works closely with the U.S. despite its links to Iran.

That intra-Shiite competition is likely to continue — sometimes violently — regardless of whether the Iraqi government and its U.S. backers force al-Sadr to disband his Mahdi Army militia or not. In military parlance, the term "special groups" refers to presumed breakaway Mahdi factions whose main sponsor is Iran.

American lawmakers expressed frustration this week because Petraeus offered no assurances that an end to the war is near.

In part, that's because the conflict has been ever-evolving — from at first a Sunni insurgency, next to a Sunni-Shiite sectarian bloodletting, and now a violent competition for power within the Shiite community.

Through much of the war, the Bush administration has presented the conflict primarily as a fight against Al Qaeda, describing it as the principal enemy in the array of Sunni and Shiite "threat groups."

That began to change after a tectonic shift in the Sunni Arab community: Thousands of Sunni tribesmen abandoned Al Qaeda in Iraq and joined U.S.-backed security forces starting last year. Attacks against U.S. forces fell sharply in former Sunni battlegrounds such as Anbar province.

U.S. troops are still fighting Al Qaeda, of course, especially in the north. Nationwide, however, most of the recent battles have involved Shiite militants.

The trouble started last month when Prime Minister Nouri al-Maliki, himself a Shiite, launched an ill-prepared offensive against Shiite militias and criminal gangs in Basra in the south.

The offensive stalled and triggered a violent backlash by al-Sadr's followers, who believed the crackdown was aimed at weakening them before provincial elections this fall.

To retaliate for the crackdown in Basra, Shiite militiamen fired rockets and mortars at Baghdad's U.S.-protected Green Zone, which houses the U.S. and British embassies and al-Maliki's office.

In the current fighting, American and Iraqi troops are trying to push the militants out of rocket range of the Green Zone and bottle them up in the sprawling Shiite district of Sadr City — the Mahdi Army stronghold.

It's a fight the Americans didn't want now. Instead, U.S. commanders believed military resources should have been continued to be directed at Al Qaeda in the north.

But now, a cease-fire that al-Sadr called last August is in tatters and the prospect of more violence looms.

The role of the "special groups" remains unclear. U.S. officials say they are breakaway factions of the Mahdi Army that no longer take orders from al-Sadr.

Such talk about the threat posed by the special groups casts the internal Shiite conflict as a proxy war between the United States and extremists controlled by Iran.

"The regime in Tehran also has a choice to make," President Bush said Thursday. "They can live in peace with its neighbor, enjoy strong economic and cultural and religious ties, or it can continue to arm and train and fund illegal militant groups which are terrorizing the Iraqi people and turning them against Iran."

But the picture is more clouded.

With all Shiite factions close to the Iranians, it appears that Iran will profit — at least to some degree — no matter which Shiite groups end up in power and no matter how America pursues the war.