They have camouflage fatigues that blend into deserts, swamps and forests. The only place they stand out is in a city - and they might well have to fight in one this time.
"You're not trying to make someone disappear, you're trying to make them obscure. There's a difference," said Timothy O'Neill, a retired Army colonel and professor of engineering psychology who helped the Marines design their new camouflage.
The new cammies, replacing 20-year-old designs, made their debut at Camp Lejeune, N.C., in January 2002 after field tests with mannequins and live troops at the Army Soldier Systems Center in Natick, Mass.
The Natick facility is also designing new cammies for the Army, but they won't be ready until 2005. The Army's current woodland fatigues have been used since 1981 and its desert uniforms were developed in 1991, shortly after the first Persian Gulf war.
Designers in this peculiar branch of military science say they study nature's own camouflage techniques, then bring in chemists, textile experts and psychologists to analyze colors, shapes and battlefield terrain in a search for ways to fool the human eye.
English camouflage scientists have spent years studying the cuttlefish, a soft-bodied sea creature with pigments that allow it to change color in less than a second.
"Almost all wildlife is camouflaged in some way," said Dwight G. Smith, a biology professor at Southern Connecticut State University.
But O'Neill, a former West Point professor whose patterns were used by NATO forces in the 1970s, said military camouflage requires an understanding of the human eye, too.
The eye, he said, routinely searches for a target by scanning an area the way a sailor scans the horizon with a spyglass, stopping to focus on any anomalies, such as a human form.
Camouflage makes it difficult to spot a target during both the scan and focus phases. To trick the eye, the splotch-like patterns must match the surrounding environment, which means they can't be too big or too small.
Color is also critical. "You don't want your colors to be too much alike," said Rick Cowan, a chemist with the Army's Natick design center.
Cowan said a uniform needs enough colors to be visually disruptive, so that the waving lines and blotches hide the outline of the human body, fooling the eye so that the shape melts into the background.
Too much of one hue will make a soldier stand out against a multicolored background - too many colors will do the same thing.
In developing the Marines' new cammies, a key was determining the terrain of future battles.
The latest fatigues were designed with the Middle East in mind, said Dee Townes, a project officer for the Corps' camouflage program.
Based on digitized photographs of potential battlefields selected by military planners, camouflage colors are identified and analyzed on a spectrophotometer, which measures reflective properties.
Scientists assign each color a number called a colormetric and create sample fatigues based on colors that will confuse the eye.
"Camouflage is both a science and an art," said Lt. Col. Gabe Patricio, who managed development of the new Marine fatigues.
Townes said the Marines' new woodland uniform is based on comparisons made of 85 types of camouflage used by hunters.
"We had heard, 'Why can't we have camouflage as good as what hunters can buy for themselves?'" Townes said.
Commercial hunters' camouflage has an advantage over military patterns because it can be designed to blend in with regional terrain and changing seasons.
The woods of Pennsylvania during November's deer hunting season have different colors than the winter terrains in Alaska and Colorado's Rocky Mountains.
"There are a tremendous number of intricate camouflage patterns out there," said Juli Case, a spokeswoman for the Industrial Fabrics Association, which represents the camouflage industry.
"You can find patterns that mimic tree bark or cattails or a forest of evergreens."
But U.S. military camouflage must clothe roughly 212,000 Marines and 480,000 soldiers.
"You can't make 20 different uniforms for every Marine or soldier," Townes said.
Planners have been calling for urban cammies since 1993, when an Army firefight with Somali militia in Mogadishu left 18 Americans dead.
But designers say making a generic urban uniform is almost impossible because cities themselves aren't uniform.
"Think of Miami, with its pinks and bright blues, and think of cities like Chicago and New York that have more gray to them," said Townes. "We're trying to get the urban mission better defined to know where to go with the colors."
The Marines' new cammies will be phased in until every Marine has them in 2006. A spokesman said there's no way to know how many Marines in Iraq have the new uniforms. About 86,000 sets of woodland fatigues and 33,000 sets of desert fatigues have been issued.
The new cammies cost about $60 a set. They're made of a cotton-nylon (permanent press to save dry cleaning costs), and are expected to last about a year.
Grunts are issued a clothing allowance to cover the cost. Officers must buy them out of their salaries.