BAGHDAD, Iraq — Even the most enthusiastic supporters of President Bush's new plan to pacify Baghdad with more U.S. troops were using phrases such as: "If it succeeds" and "If the Iraqi government lives up to promises."

Analysts were predicting extreme bloodshed and a strategic catastrophe if it fails.

The great danger for the new U.S.-Iraqi push to scour Baghdad clean of Sunni insurgents and Shiite militiamen lies in the time that has passed since Iraq fell into chaos and they began to mobilize. Both factions are better armed now and more hardened in their positions, and less likely to listen to calls for moderation and political reconciliation.

"The Shiite militias will welcome our assault and treat this as an opportunity to mobilize the entire Shiite population against us," said retired Col. Douglas Macgregor, an adviser to former Defense Secretary Donald Rumsfeld in 2003. In addition, "Iran stands to benefit most as it has thus far by seeing an eventual Shiite dictatorship emerge in Baghdad," he said in response to an e-mail query.

Steven Biddle, a Council on Foreign Relations senior fellow on defense policy, declared the plan, while containing positive elements, too dependent on Iraqi forces. The Iraqi army has proven itself unreliable in past operations and its loyalties often fall with the Shiite militiamen they will have to fight this time.

"The troop commitment announced still leaves us a long way short of what people normally... want to pacify a city the size of Baghdad, much less all of central Iraq," Biddle said. "You're going to have to rely very heavily on Iraqi security forces that have not proven up to this in the past and probably won't."

At Baghdad University, political science professor Nabil Salim said in case of failure the new plan would cause "the situation to deteriorate in a very serious and catastrophic way and the Iraqi people will pay the highest price."

Most Baghdad neighborhoods now are controlled to one degree or another by al-Qaida militants and their Sunni allies or the Mahdi Army militia of radical Shiite cleric Muqtada al-Sadr. There is a presence, but smaller in numbers, of fighters from the Badr Brigade, the military wing of Iraq's most powerful Shiite political organization, the Supreme Council for the Islamic Revolution in Iraq. The Badr Brigades were trained and armed in Iran, where its leaders were in exile until the fall of Saddam.

These groups have fought one another or the U.S. and Iraqi militaries for as long as three years and know their enemy. Through those battles, the Mahdi Army especially, has taken more and more territory in Baghdad, evicting Sunni residents from mixed neighborhoods to produce purely Shiite enclaves deep into west Baghdad which once was Sunni or mixed.

While Prime Minister Nouri al-Maliki has warned the Mahdi Army that it must finally lay down its arms or face all-out attack from his government troops backed by U.S. forces, the threat was not expected to unravel what has become a bold and very heavily armed fighting force that owes all loyalty to al-Sadr and none to al-Maliki.

Compounding the problem is the heavy infiltration of the Iraqi police, an overwhelmingly Shiite organization, by members of the Mahdi Army. Some in the organization have been trained in Iran by the elite Revolutionary Guards and their supply of weapons flow without interruption across the porous border.

There are reports that an increasing number of the group have and are trained to use silencer-equipped sniper rifles. They already have big stores of Kalasnikov automatic rifles and ammunition as well as rocket-propelled grenades and sophisticated, armor-piercing roadside bombs.

In the warren-like streets of many neighborhoods under Shiite control, including their home base of Sadr City, they would be a formidable force that could send Iraqi army forces into quick retreat, leaving embedded U.S. troops struggling in dangerous urban warfare and house-to-house combat, where the enemy knows the ground and U.S. forces don't.

The Iranians also are believed to have helped arm the Sunni insurgents, who were already receiving money and guns from the other side of Iraq, through the desert border with Syria. Huge numbers of Saddam's former Baath Party organization and his government and military fled to Syria, where they found ready refuge.

Many analysts believe that the increased number of Americans caught in such urban warfare would produce an exponential rise in U.S. casualties — far greater than the spike in the second half of last year during the most recent bid to wipe out militants in Operation Together Forward.