Insurgents opposed to the US presence in Iraq increasingly appear to have adopted a new strategy: create chaos by striking a wide range of targets.
Tuesday's suicide truck-bomb explosion at UN headquarters in Baghdad was but the latest in a string of attacks aimed at civilian and economic sites. Jordan's embassy in Baghdad was shattered by a similar bomb Aug. 7. Over the weekend Iraqi oil, water, and electricity lines were all hit by saboteurs.
The coordination involved in this campaign is unknown. US officials have said they believe that the violent Iraqi opposition is a polyglot mix of Saddam Hussein die-hards, Islamist terrorists, and criminals.
But some Western analysts believe that an influx of foreigners is driving this violence. Radical Muslims bent on jihad are now flooding into Iraq, some say, as they poured into Afghanistan during its years of Soviet occupation.
Whoever the perpetrators, their aim may be to convince the mass of Iraqis who are neither strongly anti- nor pro-American that the current situation is intolerable. They probably want to layer fear on top of the frustration and anger already felt by an Iraqi population whose economy and infrastructure are in shambles.
"The whole purpose is to demonstrate that the Americans are not in control. Nobody is safe," says a former intelligence officer with 25 years experience in the region.
The explosion at the UN's Baghdad headquarters, based in a three-story converted hotel, was the deadliest attack on an Iraqi soft target yet.
The nature of the strike showed the desperation of those who oppose the US presence in Iraq, claimed President Bush in an audio statement from his ranch in Crawford, Texas. He vowed that the US would persevere.
"The terrorists who struck today have again shown their contempt for the innocent; they showed their fear of progress and their hatred of peace," said Mr. Bush. "They're the enemies of the Iraqi people."
An enormous amount of explosives was used in the attack, according to Bernard Kerik, the former New York police commissioner now involved in the Iraqi rebuilding effort. The force of the blast ripped off the front of the building.
Early reports from the United Nations said that at least 14 people had been killed, including the top UN official in the country, Sergio Vieira de Mello.
Bystanders - the concerned, the curious, and the media - could only speculate about the nature of the blast as dusk fell in Baghdad.
One man said the bomber had been in a cement mixer whose driver was still in the vehicle as the explosion occurred.
The US military swarmed over the area in the wake of the attack, with dozens of Humvees converging on the site and helicopters circling overhead.
Those on the scene found it difficult to imagine a motive. The UN does not have a central role in Iraqi reconstruction at the moment, though it does distribute aid.
The UN's oil-for-food program, now being phased out in the wake of the US invasion, has funded regular food rations that have kept Iraqis fed for several years. Although the UN is associated in many Iraqis' minds with efforts to find and destroy Iraq's weapons, it has also maintained extensive humanitarian programs in the country.
The attack may have been "because there were so many foreigners there, probably," says Feriyal Scott, personnel director of the World Health Organization office in Baghdad. "And probably because the UN did not support the previous government [of Iraq]."
Analysts outside the country said that while they could not be sure about the reasons for the attack, it was likely the UN building was chosen almost at random because it was both a symbol of the West and vulnerable.
Blowing up the UN, as well as blowing up oil pipelines and water mains, can only make the ordinary life of Iraqis more miserable. Thus it seems obvious that the campaign of destruction aims to create chaos and deliberately harm the ability of the United States to administer Iraq.
"This could be very devastating to [US] efforts," says Judith Yaphe, an Iraq expert at the National Defense University.
It's possible that the attack was carried out by remnants of the Hussein regime, says Ms. Yaphe. Some of Mr. Hussein's elite forces, such as the Special Republican Guard, received training in car bombs and other means of sabotage.
But the smoothness of the planning and the clever choice of unexpected targets points to a more experienced kind of terrorist organization, according to Yaphe. That might mean jihadis crossing from Syria or Iran.
"I can believe there are all kinds of forces coming to play here," she says.
A US attack last month on an alleged terrorist training camp in the desert west of Baghdad killed 70 foreign fighters. They included Saudis, Yemenis, Afghans, and Sudanese, according to news reports.
A statement purportedly from Al Qaeda broadcast Aug. 18 on Arab satellite television asserted that the recent spate of attacks in Iraq was indeed the work of such jihadis.
Whatever the nature of the opposition, it is clearly adjusting, adapting, and searching out targets that have not yet been protected.
That makes life more difficult for US administrators on a number of levels. More troops might have to be dispatched for the guarding of pipelines and other infrastructure, for instance - stretching an already thin force.
Pressure to send yet more soldiers to Iraq might increase. On Tuesday, Sen. John McCain (R) of Arizona said publicly, "I think they need more people."
Mr. McCain did not specify how many additional troops he thought were necessary. Current US end strength in Iraq is about 140,000.
At the same time, nations that the US is attempting to persuade to join the occupation effort may become more reluctant to get involved. India, Pakistan, and other such countries might be wary about sending their units into a clearly hostile country.
"Most have wanted to avoid this for obvious reasons, and this is going to make it [easier for them to do so]," says Jim Walsh, an international security expert at Harvard's John F. Kennedy School of Government.
The real target of the blast was not so much the UN as the Iraqi people at large, according to Mr. Walsh. Without their confidence, the US occupation of Iraq won't succeed in building a stable nation.
"It's hard to get [the insurgents] unless the average Iraqi has faith and ... a sense of security and sees their future best hope is for a successful reconstruction," says Walsh.
But a simmering summer with poor electrical service and 60 percent unemployment has already hammered Iraqi morale. US military tactics that seem as occasionally heavy-handed may only become more so as the search for Hussein adherents and terrorists intensifies.
Both the Shiite and Sunni Muslim religious establishments have taken umbrage at recent US actions, say other experts. The establishment of an Iraqi governing council may have been a step in the right direction, but, like US administrators, the Iraqi leaders have to operate from behind barbed wire and a guard of US guns.
"I don't think we are poised to break out of this very quickly," says the former intelligence officer with expertise in the region.
• Cameron Barr contributed from Baghdad.