Breathing hard and leading with their rifles, a cluster of US marines takes cover behind a mountain of rubble. Another team dashes across a field, concealing themselves behind a large metal wheel.
"Rat-a-tat-tat!" shouts one, like a comic-book warrior. "You're dead!" declares another, at pretend insurgents.
If training is key to battlefield success, the marines of the 1st Marine Expeditionary Force are trying to maximize their skills as they prepare for the type of urban offensive in Fallujah that most military experts anticipated 18 months ago, when US forces first entered Baghdad.
Senior US and Iraqi officials say an invasion of the city could begin within days, in a bid to decapitate the insurgency that has spread across Iraq. The challenge for these troops will be to stay one step ahead of a resistance that is constantly evolving, has become adept at using the Internet to share tactics, is fighting on its home turf, and has had months to prepare.
"They are changing all the time - it's cat and mouse, and we're trying to stay the cat," says Capt. Gill Juarez, an armored company commander from San Diego.
Some 52 Marines died in Fallujah and elsewhere in Iraq last April, when a US invasion force entered the city, igniting resistance across a string of cities.
"Their only advantage is they can use the asymmetric threat, and we can't go there, nor should we," says Captain Juarez, referring to guerrilla tactics that include roadside and suicide car bomb attacks, which killed eight marines near Abu Ghraib last Saturday.
On Juarez's desk in a Spartan makeshift operations center, is a worn book called "Russia's Chechnya Wars 1994-2000: Lessons from Urban Combat." The light armor commander has asked all his officers to read it, to understand how Russia's superior firepower - brought to bear destroying the Chechen capital, Grozny - did not bring victory.
"It's no secret for mechanized units: you're vulnerable [in urban warfare]," says Juarez. "You have to have dismounted security. I tell my scouts [marines on the ground] that they need to have eyeballs 'one feature over.' They need to know what's going on the other side of the wall or the berm."
Despite the mustering of US Marine and Army forces for any Fallujah invasion, and thousands of newly trained Iraqi troops to control the city afterward, not all in Iraq's interim government are convinced about the tough approach.
Ghazi al-Yawar, Iraq's interim president, criticized the effort, saying in an interview with a Kuwait newspaper that the standoff called for "continued dialogue." And Mohammed Bashar al-Faidhi, spokesman of the Association of Muslim Scholars, warned Tuesday that an assault on Fallujah would spur his Sunni clerical group to use "mosques, the media, and professional associations" to proclaim a civil disobedience campaign and a boycott of the January elections. Two Iraqi cities also saw fresh violence Tuesday as a car bomb in Baghdad killed at least six people, while in Mosul, a car bomb exploded near a military convoy, killing four civilians and wounding at least seven soldiers.
A further challenge in Fallujah, US commanders say, is the apparent ease and speed with which insurgents have adapted their tactics. And this offensive will be no surprise to Fallujah - the showdown has been telegraphed for weeks; more than 80 percent of the population of about 300,000 are believed to have left the city to avoid the invasion.
"They've had a lot more time to prepare," says Lt. Col. Michael Ramos of Dallas, a battalion commander. If past operations are any measure, the marines are likely to face an array of roadside bombs, booby traps, and other surprises.
Every vehicle in the city will be considered a potential car bomb; every person who remains, a potential insurgent. Guerrillas spread their expertise on the Internet and word of mouth throughout their strongholds. "The use of technology is really changing the face of warfare," says Colonel Ramos. "The speed with which you can spread an idea is so fast. The loop between action and innovation is getting smaller and smaller."
Despite those changes, the battle for built-up Fallujah is expected to be an infantry fight. And some insurgents still fight the old way, even against tanks.
"We've seen the enemy come running at our tanks with small arms fire," says Capt. Robert Bodisch, an M1A1 tank company commander, who adds that his units "every day reduce the insurgent population of Fallujah."
"I think they honestly believe they can damage us," says Captain Bodisch. "But then they are not around long enough to go back and tell their buddies." Still, the tanks are "not really designed to fight in an urban environment," the captain says. "We've had to change our tactics."
Civilians in the city complicate the picture, and have recently limited the marines' ability to shoot. While any battle for Fallujah is likely to err on the side of more firepower, officers have shown the marines they are serious about strict rules of engagement.
When a civilian was shot dead and three wounded by marines early Monday at a checkpoint, where mixed signals to stop or go appear to have been sent to the Iraqi driver, investigating officers took the unit aside and questioned individual marines within hours of the incident.
But tactics are what now occupy the marines, as they work to integrate armor and infantry for battle. They have even refitted their gas masks, armed with longstanding intelligence about a potential threat of a poison-gas attack.
The dry runs with combat units this week are yielding lessons, and ironing out issues before they turn into battlefield problems. "Good job on concealment - you guys were up a little bit too long," says Cpl. Steven Komin of Mundeleine, Ill., standing on a crushed concrete platform to address the fire teams after the combat drill. "You've got to move quickly - sometimes you won't have cover at all; just work with what you have."
"We're looking forward to this," says Lance Cpl. Geoffrey Bivens of Katy, Texas, one of the marines doing the mock urban combat training. "Now [insurgents] are really scared to come close to us. They know it's suicide."
"They're just spraying and praying," says Lance Cpl. Lance Fischer, of Bradenton, Fla., dismissing insurgent rifle fire. "They're not trained marksmen, like we are."
That sparked a warning from Corporal Bivens: "Yeah, but they're spraying and praying at chest height."
• Material from the wires were used in this report.