She warns her men that insurgents have been reinforcing, that on the previous day two suspected roadside bombs were discovered on our route, that US troops had “lit up” (destroyed) an Iraqi car as it sped towards a convoy. Watch out, she says, for a hijacked fuel tanker that could be turned into one huge firebomb.

The soldiers form a circle and bow their heads as Sergeant Malcolm Smith from Alabama leads a prayer. “Oh God our Father, we pray for your continual blessing upon us as we travel along this road,” he intones.

“We pray for a safe journey to our next destination . . . If it is your will that we encounter the enemy, give us the courage to use what we’ve been taught and bring it back to our remembrance.” The men then stretch their hands to the middle and shout, as one, the company nickname: “Heavy Truck”.

Soon Lieutenant Constantino’s mile-long convoy is rumbling north across the dusty no man’s land that divides Kuwait from Iraq — a modern-day wagon train heading into some of the most hostile territory on earth. Richard Mills, the Times photographer, and I are on it, not least because this is the only way that Western journalists can still travel overland across Iraq. It proves to be quite a journey — and one that ends with a bang.

Every day the US military sends up to 20 convoys from Kuwait to the 55 US bases scattered across Iraq. Each day they deliver roughly 12,000 tonnes of supplies to 200,000 US troops and civilian workers. That makes it the largest military sustainment operation ever mounted over a protracted period.

It is also extremely dangerous. The long, lumbering convoys are regularly attacked with roadside bombs, rocket-propelled grenades and small arms fire. “Everybody goes out with the expectation that they could be shot at or hit by an IED (improvised explosive device),” says General Thomas Robinson, commanding general of the 377th Theatre Support Command, which runs the convoys.

Support troops used to operate well behind frontline troops, but in Iraq the front line is everywhere. The 377th had ten soldiers killed in action last year alone. The casualty rate among the private security guards who escort many of the convoys, and the “TCNs” (third country nationals) who drive many of the lorries, is far higher.

There are no official figures, but a partial list compiled by the website icasualties.org shows that 156 contractors have been killed in convoy attacks, ambushes and roadside bombings. Four Americans and one Austrian are still missing after their convoy was ambushed near the Kuwait border last November.

There is no other way of sustaining so many troops thousands of miles from home. It would be impossible by air alone. But the supply chain from Kuwait is a slender lifeline, and it could soon become more dangerous still.

President Bush has promised to use the extra troops he is sending to Iraq to crack down on Moqtada al-Sadr’s Mahdi army in Baghdad. The Mahdi army could easily retaliate by attacking more convoys travelling up through the Shia south. Moreover, US officers see Iran’s hand in some of the convoy attacks, especially in a lethal new IED called an explosively formed projectile that penetrates the thickest armour.

The Times’s journey began in a military camp outside Kuwait City with a briefing from Colonel Sam Pearson, a 50-year-old human computer who is director of the 377th’s Distribution Management Centre and the man at the nerve centre of the sustainment operation.