'The single most effective weapon against our deployed forces'

Insurgents often post video clips of their attacks on the Internet, the equivalent of taking scalps. They also exploit the Web -- either openly or in password-protected sites -- to share bomb-building tips, emplacement techniques, and observations about American vulnerabilities and countermeasures.

For example, a 71-page manual titled "Military Use of Electronics Prepared by Your Brother in Allah" was posted on a jihadist Web site earlier this year. Comparable in sophistication to an introductory college electrical engineering class, the manual provided color photos and detailed diagrams on "remote wirelessly operating circuit using a mobile phone for moving targets" and "employing timers to explode detonators using transistors."

The lack of success in combating IEDs has left some military officials deeply pessimistic about the future. "Hell, we're getting our ass kicked," said a senior officer at U.S. Central Command, which oversees the wars in Iraq and Afghanistan. "We're watching warfare that's centuries old being played out in a modern context and we're all confused about it. The toys and trappings have changed, but asymmetric fighting, and ambush, and deceiving and outwitting your opponent, and using the strengths of your opponent against him, are ancient."

Others point to several heartening developments. The number of IED attacks declined in Iraq late this summer after five more U.S. brigades took the field as part of a troop "surge" ordered by the White House. American casualties from IEDs also dropped. Throughout Iraq, more than half of all makeshift bombs are found before they detonate.

Moreover, improved body and vehicle armor, as well as sophisticated combat medicine, mean that the proportion of wounded U.S. soldiers to those killed in Iraq is about 8 to 1, a survivability ratio much higher than in previous wars. Also, about 70 percent of wounded soldiers return to duty within three days, according to Pentagon figures.

"We've saved a lot of lives," Deputy Defense Secretary Gordon R. England said in an interview last month. "We've had people killed and injured, but we've probably saved five or 10 times that number of people by preventing attacks, or capturing and killing [insurgents], or getting caches of weapons, or disabling them."

In 2003, almost every IED caused at least one coalition casualty. Now, Pentagon figures indicate, it takes four of the bombs to generate a single casualty. In addition to more aggressive attacks against IED networks, rather than simply defending against the device, various technological advances have shaped the battlefield.

The military, for example, now has about 6,000 robots, compared with a handful four years ago. And bombs detonated by radio-controlled triggers, which had become the most prominent killer of U.S. forces, today amount to only 10 percent of all IEDs in Iraq after the deployment of 30,000 jammers, with more on the way.

Still, as a "Counter IED Smart Card" distributed to American troops warns, "In Iraq, nothing is as it appears." The cycle of measure, countermeasure and counter-countermeasure continues.

Two particularly deadly IEDs now account for about 70 percent of U.S. bombing deaths in Iraq: the explosively formed penetrator, an armor-killing device first seen in May 2004, and linked by the U.S. government to Iran, and the "deep buried," or underbelly, bomb that first became prominent in August 2005.

Grievous as the IED toll has been on U.S. and coalition forces, the impact on Iraqis is greater. The Pentagon considers an explosion to be "effective" only if it causes a coalition casualty; this reflects a judgment that the strategic impact of an IED derives from its ability to erode American will, which in turn is predicated on casualties suffered by U.S. troops or their non-Iraqi allies. By this yardstick, the suicide truck bombs that killed more than 500 civilians in northwest Iraq on Aug. 14 of this year are considered "ineffective"; so, too, the IED on Sept. 13 that killed a prominent sheik in western Iraq whom President Bush had publicly praised a week earlier for his opposition to al-Qaeda extremists.

But few military strategists doubt that Iraq's future depends on reducing IED attacks of all sorts. "If you can't stop vehicle-borne IEDs from being detonated in public spaces, you can't build a stable society," a Navy analyst said.

No one is ready to declare the dip in the number of bombs this summer to be an enduring decline. Insurgents appear "able to put out more IEDs to maintain that constant level of death-by-a-thousand-cuts," a senior Pentagon analyst said. "We have not seemed able to put an upper bound on that number."

And there is another mostly unspoken fear. With approximately 300 IED attacks occurring each month beyond the borders of Iraq and Afghanistan -- a Pentagon document cites incidents in the Philippines, Russia, Colombia, Algeria and Somalia, among other places -- the question occupying many defense specialists is whether the roadside bomb inevitably will appear in the United States in significant numbers. "It's one thing to have bombs going off in Baghdad, but it will be quite another thing when guys with vests full of explosives start blowing themselves up in Washington," said the Navy analyst. "That has all sorts of repercussions, for the economy, for civil liberties."

For now the device remains an indelible feature of the Iraqi and Afghan landscapes. "The enemy found a seam," said an Army colonel. "I don't think they knew it was a seam, but it just happened."

Staff researchers Madonna Lebling and Julie Tate in Washington contributed to this report.

Read Part 1: Summer 2002 to Summer 2004