The holes gouged from the pavement were a reminder, as if one were necessary, of the danger to Bravo Company as it rolled through the shimmering heat.

"Stop here," Lt. Vince Noble said quietly. His Humvee, guiding three others, eased to a halt 30 yards from a bridge. Noble, in the right-hand seat, peered through his field binoculars. He lingered, examining the dirt near the bridge with the care of an archaeologist. He was looking for a subtle change in color, scrape marks, any evidence of recent digging. Or, even more telltale, the wisp of the antenna of a walkie-talkie that would set off a roadside bomb.

Bravo Company's job this day, and every day, was to keep the roads open in this piece of western Iraq. Roadside bombs and kidnappings of truck drivers threaten to throttle Iraq, which depends on the steady caravans of trucks hauling in everything from food to fuel and furniture.

Insurgents know the roads are vulnerable. Last week, one group issued a videotape threatening to cut off the main highway linking Baghdad and Amman, the capital of neighboring Jordan. It did not happen, but truckers are nervous. Nearly a dozen have been kidnapped; more have been killed by roadside bombs. In the southern city of Najaf last week, more than 30 truckers refused to move without a large guard escort.

Louei Hatim Aris, the interim transportation minister, is creating a special police fleet to accompany truckers, with mounted machine guns in the front and back of each convoy; armed men would ride in every truck. But while that might deter kidnappings, it will not stop roadside bombings. That task still falls mainly to American forces such as Bravo Company, attached to the 7th Marine Regiment in western Iraq.

"We have to keep the civilian and the military convoys moving. We have to make sure whatever is on that convoy gets through," said Maj. Mark Winn, 41, the executive officer of the regiment's 1st Battalion. "We also have to show we are in control of the area. If you lose vehicles every day, pretty soon you are going to start losing civilian drivers who are willing to drive that route."

The effort might seem personal to officers here. In one 48-hour period, the top three regimental commanders were hit by roadside bombs, Winn said. All escaped serious harm, though Col. Craig Tucker, the top commander, spit shrapnel out of his mouth that came through his cheek, according to Winn.

The bombers prefer military convoys, adding to the toll of U.S. casualties by grim ones and twos, but they also target truck convoys. A top official of the interim Iraqi government argues that roadside bombs are a sign of the opposition's desperation.

"The terrorists have changed their tactics, from openly confronting the security forces to using kidnapping and bombs," Georges Sada, a spokesman for the interim prime minister, said last week. "For sure, it's a cowardly thing. But it's easy to take someone from the highway or set a bomb at night."

Many Iraqis assert that the spectacular suicide bombings that often kill by the dozen are being carried out by foreigners. "Iraqis don't kill themselves," said one government security official. But the roadside bombs, they admit, are probably the work of Iraqis who were loyal to ousted president Saddam Hussein and want to disrupt the new government.

For the U.S. military, the search is a deadly game of move and countermove. As the Americans change their tactics, the bombers invent new threats. U.S. forces used to see roadside bombs that were set off by a switch at the end of a long wire. But when they started tracking those wires and killing the triggermen, the bombers began using remote detonators: garage door openers, toy car remote controls, wireless doorbells and cell phones. A favorite is a simple walkie-talkie -- the kind that can be bought at a Radio Shack outside Iraq -- wired to a detonator. It can be set off from two miles away.

Noble, 26, a Naval Academy graduate from Philadelphia, said the bombers recently have begun leaving explosives in cars that blend in with the ubiquitous broken-down vehicles beside Iraqi roads.

On a recent morning, men from Noble's company kneeled around him as they planned the day's patrol. They already were sweating in the fierce midmorning sun. In the open-top Humvees, they got hotter and dirtier.

They rolled out of their camp near Haditha, past the sign saying "Complacency Kills," onto Highway 12, a major road that reaches through the brown desert from Baghdad to the Syrian border.

In the lead Humvee, Lance Cpl. Mike Riggle, 21, from Youngsville, Pa., steered to the center of the road, away from the right side, where bombs would be placed. He edged over only to let traffic pass. All eyes in the convoy swept the roadside for suspicious scenery.

"We've been up and down this road so many times we can tell what pile of dirt is new," said Sgt. Shawn Gianforte, 27, of Caledonia, N.Y.

Black-faced sheep and scrawny goats drifted into the road, chased back by shepherds with long switches. Noble's Humvee, the most likely to get hit because it was first in line, was enclosed in armor; those trailing had steel plates bolted to them. "In theory, they protect us from shrapnel," said Gunnery Sgt. Kristian Eckholm, 33, of Green Bay, Wis. That was in case the convoy "found one the hard way."

A trail of ragged holes in the highway showed the work of the bombers. At one spot, three craters 10 yards apart were evidence of a "daisy chain" -- bombs linked together to try to hit several vehicles in the convoy.

Bridges are favored spots for the roadside bombs, and Noble approached each one with caution. At one bridge, where two craters wrote the signatures of previous bombs, Noble ordered his men out of the Humvees to search under and around the structure. They fanned out quickly, and several of the Marines knelt and aimed their weapons at the brown ridges nearby. They peered through their scopes for any sign of a triggerman.

Scrambling along an embankment underneath the bridge, Cpl. Daniel Vella found no bombs but noticed Arabic writing on a steel beam, accompanied by a sketch of a truck. "We'll have to come back with an interpreter," Noble said.

Four hours later, Noble guided his men back to his camp, hot and tired. No bomb had been found, and he noted, "When it's quiet, it means we're successful."

U.S. Marines fan out as they search a roadside under the hot Iraqi sun for evidence of bombs, such as recently moved dirt or the tip of a walkie-talkie antenna.Marine Lt. Vince Noble, left, listens to a report from Lance Cpl. Mark Roop while scouring for bombs along a road from Baghdad to Syria.