QUANTICO, Va. -- Here at "the crossroads of the Marine Corps," some officers are uneasily pondering a paradox: No service was better prepared than the Marines for the challenges of post-invasion Iraq, yet no service has found its mission there more unsettling to its sense of itself.

When asked in 1997 to describe the kind of conflict for which Marines were training, Gen. Charles Krulak, then the Corps' commandant, replied with one word: "Chechnya." He meant ethnic and sectarian conflict in an urban context. He spoke of "the three-block war" in which a Marine wraps a child in a blanket, then is a buffer between warring factions, then engages in combat, all within three city blocks.

For Marines, however, fighting such a war for more than four years jeopardizes the skills essential to its core mission -- combat as an expeditionary force. Marines have not conducted a major amphibious landing since Inchon in Korea, but the Corps, which specializes in operational maneuver from the sea, remains, in theory, a force that penetrates, performs, then departs. Marines say: The nation needs the Army, Navy and Air Force, but it wants the Marine Corps as an expeditionary power, more than just a miniaturized Army.

Marines have an institutional memory of "small wars," from the Philippines to Central America, and this competence serves them well in Iraq, which is, an officer here says, "a thousand microcosms." But the exigencies of the protracted Iraq commitment have forced the Marines to adopt vehicles that are heavier and bigger than can easily travel with an expeditionary force on ships. And there is tension between the "nation-building" dimension of the Marines' Iraq mission and the Corps' distinctive warrior (BEG ITAL)esprit(END ITAL), which is integral to why the nation wants the Corps.

Officers studying here at the Marine Corps University after tours in Iraq dutifully say they understand that they serve their combat mission -- destroying the enemy -- when they increase the host nation's capacity for governance. Besides, says one officer, when his units are helping with garbage collection they know that "garbage collection is a matter of life and death because there are IEDs (improvised explosive devices) hidden under that garbage."

Still, no one becomes a Marine to collect garbage or otherwise nurture civil societies. And as one officer here notes with some asperity, there is "no Goldwater-Nichols Act for the rest of the government." That act mandated "jointness" -- collaborative operations -- by the services. Civilian agencies that do not play well together have fumbled the ball in Iraq, and the military has been forced to pick it up. This draws the military deeper into the sensitive responsibility for tutoring civilians who assign the forces nonmilitary tasks.

The political dimension of leadership training remains, however, secondary to instruction in military valor. The other services tend to teach leadership prescriptively, with rules. The Marines teach descriptively, with storytelling about what happened on the sea wall at Tarawa (1943), at Korea's Chosin Reservoir (1950), in Vietnam's Hue city (1968). But there is another story pertinent to providing military advice that can assure civilian comprehension of military functions.

Early in the Kennedy administration, when there was talk about a U.S. invasion of Cuba, Gen. David M. Shoup, Marine commandant, gave President John Kennedy and his advisers a tutorial. David Halberstam wrote in "The Best and the Brightest":

"First he took an overlay of Cuba and placed it over the map of the United States. To everybody's surprise, Cuba was not a small island along the lines of, say, Long Island at best. It was about 800 miles long and seemed to stretch from New York to Chicago. Then he took another overlay, with a red dot, and placed it over the map of Cuba. 'What's that?' someone asked him. 'That, gentlemen, represents the size of the island of Tarawa,' said Shoup, who had won a Medal of Honor there, 'and it took us three days and 18,000 Marines to take it.'"

Because of the dispersed battlefield in Iraq, company commanders must make instantaneous decisions that battalion commanders used to make, and corporals are making decisions that officers used to make reflecting -- and affecting -- the Marine Corps' ethics and core values.

Still, "it's a beautiful thing being in Iraq," says one officer, "because you aren't worrying about Corporal Jones stateside getting a DUI." That is the durable voice of the Marine Corps, which is "first to fight," and is happier when doing so than when dealing with garrison duties stateside or chores properly belonging to civilian agencies abroad.