Months before the invasion of Iraq, Pentagon war planners anticipated the fall of Saddam Hussein would usher in a period of chaos and lawlessness, but for military reasons, they chose to field a light, fleet invasion force that could not hope to quell such unrest when it emerged, Pentagon officials said yesterday.

Follow-on forces such as the 4th Infantry Division and 1st Armored Cavalry Regiment were expected to carry the brunt of what Pentagon planners call "stabilization" duties. But Baghdad fell so quickly that those troops could not deploy fast enough to head off widespread looting. Military police officers scattered throughout the Army and Marine combat vanguards were trained to handle massive numbers of enemy prisoners of war, not civilian unrest, defense officials said.

President Bush and senior Defense Department officials responded indignantly yesterday to criticism from human rights organizations, Arab government officials and some ordinary Iraqis that military forces have done far too little to stop the looting that has stripped government ministries, museums and hospitals clean.

"You know, it's amazing," Bush said. "The statue [of Hussein] comes down on Wednesday and the headlines start to read: Oh, there's disorder. Well, no kidding. It is a situation that is chaotic because Saddam Hussein created the conditions for chaos. He created conditions of fear and hatred. And it's going to take a while to stabilize the country."

Those chaotic conditions have not come as a surprise, some administration officials said, nor are they necessarily all bad.

Richard Perle, an influential Pentagon adviser, said some amount of blood-letting may have to be accepted until U.S. and newly installed Iraqi authorities can develop a legal system to purge Baath Party members from Iraqi society. Perle raised the specter of the French Revolution's Reign of Terror and its leader, Maximilien Robespierre. "The Iraqis know who their oppressors are in their midst," he said. "It isn't Robespierre, I hope. But it's up to the Iraqis, and there is bound to be score settling. If you know Rashid worked at the place where your brother was tortured and killed, people can be forgiven for chasing down and killing Rashid."

Douglas J. Feith, the undersecretary of defense for policy, was quick to say such vigilantism is not U.S. policy. "It may happen," he said, "but it is not something that we want to happen."

Feith said administration officials anticipated such unrest months before launching the invasion. Pentagon war planners -- hoping to plot out a course toward stability in a post-Hussein Iraq -- have been consulting with Eastern Europeans and South Africans who were involved with their countries' transitions from authoritarian rule and the establishment of truth and reconciliation commissions, he said.

The mix of forces reflected some understanding of potential unrest. Col. John Della Jacono, chief of staff for Coalition Forces Land Component Command in Iraq, told reporters last week that all allied military units have military police attachments, who were prepared to whisk 50,000 or more prisoners of war off the battlefield and ferry them to a detention facility at Umm Qasr on the Persian Gulf. Instead, only 6,200 Iraqis were captured.

But even though MPs had far fewer prisoners to deal with, they could not possibly be expected to immediately assert a police presence while a relatively light U.S. and British force was stretched from the Persian Gulf to the far north of a country the size of California, Pentagon officials said. That is especially true when they are taking sporadic fire from Hussein loyalists and launching an assault on the deposed strongman's birthplace of Tikrit.

"There is still a war on," Deputy Defense Secretary Paul D. Wolfowitz told Muslim journalists Friday. "There is still even the possibility of chemical or biological weapons being used. There is still execution squads out trying to kill Americans, to kill civilians, to kill Iraqis. . . . The expectation that these issues would be resolved at this point is totally unreasonable."

Some Iraqis may be asking for U.S. forces to quell the unrest, but others would almost certainly object to the results, Perle noted. "I don't think it could have been readily suppressed without using violence against looters of a scale our soldiers would have been loath to use," he said.

Baghdad collapsed so quickly that U.S. forces did not have a chance to establish civil authority in parts of the country already occupied, and a defense official said no amount of planning can predict the specific stabilization requirements that are determined by battle events -- bomb and combat damage -- and post-battle events such as looting and vigilantism. "We have to see what the requirements are and set about fulfilling the requirements," the defense official said.

Vanquished police and military officers must be vetted to determine which ones could be brought back to their posts and which ones would be considered instruments of repression. U.S. officials are also considering the proper mix between police officers, regular army troops, Republican Guard troops and newly recruited Iraqis for a stabilization force.

The wholesale plundering of Baghdad and other major Iraqi cities did begin to subside yesterday, as civilians combated looters on their own and U.S. troops became a more assertive presence, especially around Baghdad's banks and hospitals, most of which have been looted.

But sporadic violence continued to flare. Looters stormed the Al Salam presidential palace on the western bank of the Tigris River, taking bone china, luxurious wash basins and bath tubs, and even fish from the garden pond. Graffiti appeared on a Baghdad wall, scrawled in English: "Bush supports looters."

Armed men surrounded the house of the Grand Ayatollah Ali Sistani, a top Shiite Muslim cleric in the Iraqi holy city of Najaf, giving him 48 hours to leave the country or face attack, according to Reuters. At the urging of Washington, Sistani had issued a fatwa calling on Shiites not to resist U.S. and British troops.

The first reports of blood-letting emerged from northern Iraq, where Arabs once loyal to Hussein and U.S.-backed Kurds told reporters that at least eight had died in running battles going on between the oil center of Kirkuk and Tikrit.

Such reports have tarnished the U.S. military's victory over the Hussein government and have raised sharp questions over the United States' moral and legal obligation to quell the violence.

"It was eminently foreseeable that there would be this security vacuum before the complete end to hostilities," said Diane F. Orentlicher, professor of international law at American University. In the case of this particular war, she added, the occupying power's legal obligation to maintain law and order under the Fourth Geneva Convention on the laws of war is augmented by "a moral obligation, given that this was an elective war, justified on humanitarian grounds."

Peter J. Spiro, an international law expert at Hofstra University, said administration officials could legitimately assert that the United States is not yet an occupying power as defined by the Geneva Convention because its troops are still engaged in combat. But, he said, by the middle of this week, the excuses for not establishing peace and order, electricity and other basic needs will begin to wear thin from the perspective of international law.

Deputy Secretary of State Paul D. Wolfowitz: "There is still a war on," and the expectation that the chaos in Iraq could be controlled by now "is totally unreasonable."