'There was a two-year learning curve . . . and a lot of people died in those two years'

Lt. Gen. Smith also advocated a "left of boom" strategy that would attack those IED networks long before a device detonated. He was appalled that many field commanders seemed to accept bomb casualties as an inevitable cost of fighting the insurgency. "We have got to go after the system," Smith urged. "We have got to find out where they're made."

But "the system" was amorphous, shadowy. Battling the IED itself, particularly through heavier armor, electronic jammers and better patrol tactics was tangible and urgent, even if it remained "right of boom, or right at boom," as a senior Army officer put it.

"It's so easy to just defend against the device, because that's what's cleaning your clock," a top Defense Department scientist said later. "We needed to move to the left of boom, but we didn't know how to do it." One briefing chart included a picture of a light bulb with the caption, "Does anybody have a bright idea?"

Abizaid had asked the Pentagon for "a Manhattan Project-like" effort to fight IEDs, which he called "my number one threat in Iraq." Like Smith, he worried that too many soldiers looked at roadside bombs as they did snipers or other vexing battlefield threats, without realizing that rising bomb casualties eroded American support for the war. Paradoxically, IEDs were "a strategic issue," Abizaid observed, "but not necessarily a tactical issue."

Whether the nation could conjure an IED solution, as the Manhattan Project had delivered the atomic bomb in 1945, also remained uncertain, given how little of the country seemed mobilized for war. "We asked for the Manhattan Project," Abizaid would later joke, "and we got the Peoria project."

Through the winter of 2004-05, bombs grew more lethal and bombers more ingenious. Explosives often were "boosted" with propane or gasoline canisters. Triggers now included pressure plates, walkie-talkies and long-range cordless phones with a range of three to five miles -- common in Iraq, where no Federal Communications Commission restricted their usage. In December 2004, insurgents in Baghdad lured police into a house, then detonated an estimated 1,700 pounds of explosives, killing at least 28 people, according to a U.S. Army War College study. Such death traps soon were known, inelegantly, as "house-borne" IEDs.

When Marines fought to reclaim Fallujah in November 2004, explosives experts examining a Euphrates River bridge discovered artillery shells hidden behind the metal base plates of light stanchions, with a triggering wire leading to a palm grove. Other rigged shells were found in drainpipes, flowerpots and phony, hollow curb stones made from plaster molds. A Marine regimental commander later recalled wondering, "What are we getting into?"

Car bombs, which had averaged one per week in Iraq in January 2004, kept doubling and redoubling. From September to December, 247 "vehicle-borne" IEDs targeted coalition forces, who learned to watch for the sagging automobile suspensions that might indicate a trunk packed with artillery shells. Mayhem often brought political consequences: After an explosion in Iraq killed eight Ukrainian soldiers in early January 2005, Kiev announced it would withdraw its 1,600 troops from the war by midyear.

A counter-IED field manual compiled by the Army and Marines warned of devices placed in "fake bodies or scarecrows in coalition uniforms." Known to bomb squads as "come-ons," such lures soon used real bodies, including IEDs tucked into the chest cavities of dead Iraqis, whose corpses also were booby-trapped with anti-tampering triggers, according to a Navy explosive ordnance disposal officer.

Insurgent bombmaking prowess derived largely from three former Baathist organizations, which had slipped underground after the 2003 invasion, according to a senior Defense Department official: the Special Republican Guard, the Special Security Organization and the so-called M-21 directorate of the Mukhabarat intelligence service. In the fall of 2004, Centcom drafted a list of 38 suspected insurgency leaders, most of whom were living in Syria, a former top Pentagon official said.

As the Cerberus Project had warned, the bombmaking "knowledge base" soon expanded, with pockets of expertise throughout Iraq. "The number of networks has been placed at 169, but that may be just 169 that we know about," the DOD official said last spring. "Some of them are Sunni rejectionists, some are Sunni Baathists, some are al-Qaeda, some are Shiite. It's a crazy quilt."

A recent intelligence study described fluid, decentralized cells, typically with five to 10 members, including a financier, a bombmaker, an emplacer, a triggerman and often a video cameraman. Some freelance bombmakers "have posted their contract for hire services through the Internet with video footage of past acts serving as promotion and bona fides," the study noted.