Killing people was never the hardest thing. You got numb to that. The hardest thing was giving an order that might result in the death of somebody under you, somebody you slept next to in a poncho hootch, or maybe and with in a bunker while red and gress tracers lit up the blackness and mortars and B-40s whumped and spattered above you. Still, there were times . . .

"I remember the first guy I knew I killed. I got him with a blooper, which was our name for a 40-millimeter grenade launcher. I'd been over there only a couple of months. This guy with me reached down and pulled out the dead guy's wallet. There was a letter from his wife in it. He was just some poor little Vietnamese who lived back in the mountains. They guy tossed up the wallet and said, 'Here, Lieutenant, maybe you want to write a letter to his wife.' That got to me."

In 1969, 1st Lt. James Webb of Delta Company, First Battalion, Fifth Regiment, U.S. Marines, was spending his days in the An Hoa Basin of Vietnam, "taking rounds," and of course giving back more than a few of his own. Like that time a bald, grinning head popped out of a thatched hole two feet in front of him and, BARROOOM, Webb blew it away point-blank with a .45 before anyone around him could pay dinky dau (Vietnamese for crazy, not yet knowing there was a small, oval object hissing just to his left.

The An Hoa Basin of Vietnam is a place of red dust and furnace heat, of wide fields broken by little green ponds, of bomb craters and pagodas and gravestones. Two rivers join there, at Liverty Bridge. Down the way are Charlie Ridge and the Arizona Valley. In its way, says James Webb, the place is beautiful.

James Webb has written a novel about Vietnam and the An Hoa Basin. The novel is called "Fields of Fire" and it is possibly the best combat book yet written about that failed war. It is not literature in the sense of, say, "The Naked and the Dead"; it is a straighforward read with its own kind of firepower - a brutal glimpse, says an advance review in Publishers Weekly, "of the absurd hell it must have been."

The other day, James Webb sat in a restaurant on Capitol Hill, down the street from his job with the House Veterans Affairs Committee. His sleeves were shoved up and his spare, toned body, connected to its thick neck and boyish - almost adolescent - face, was edged forward on his chair. He looked erect and expectant, an officer out of uniform.

At one point, talking of those who managed not to go to Vietnam, he said, slowly: "I just want the Vietnam veteran to be accorded the same understanding." He stopped and smiled. "We haven't had our catharsis yet."

James Webb, who is 32 and a lawyer how, was medically separated from the Marine Corps in April 1972. By then he had been meritoriously pronoted to captain and had been decorated with the Navy Cross, the Silver Star, two Bronze Stars, the Navy Achievement Medal. He had also been operated on three times. The last doctor put it to him this way: If he stayed in, he'd have a straight leg in three years.

One day Webb went down to Florida to see his folks. The three of them, Webb and his mother and father, were driving through an orange grove. Suddenly somebody behind or to the side fired a gun. Involuntarily, reflexively, James Webb, emitted a strange, high-sound - a kind of "unhhh," he says, half cry, half drowning gurgle. It is the sound men under fire sometimes make unconsciously. Webb's father, a retired career Air Force officer, turned to his wife and said: "You see."

Jame Webb has lots of stories like that. Part of the reason for "Fields of Fire," he says, was to get some emotion, not politics or morals, out on paper. He wanted to give the texture of an experience, render it if he could apolitically.

James Webb is in the novel, of course - as a composite of characters named Snake and Robert E. Lee Hodges and Doc Rabbit and Cat Man and Bagger and Homicide and Speedy and Senator, who was not really a senator but had had three years at Harvard. Webb himself had four years at the Naval Academy, where he took "literature to relieve courses on the thermodynamic properties of steam." He never really thought of himself as a writer.

Usually when James Webb talks about the war, he is talking about people, men who served under him and whom he still can't quite get out of his head. "I see these guys and I see what's missing in their lives," he says. He says it stoically, as he says a lot of things.

Mike McGarvey, known to his outfit as "Mack," was a soldier who served under James Webb. Mack is now a one-armed, long-haired, over-weight, Harley-Davidson motorcycle mechanic in Nashville. "You talk about your one-armed paperhanger," Webb laughs. Webb was there the day they medevacked Mack out of the bush, when a corpsman picked up his blown-off shoulder, and dropped it on the poncho where he lay shot up with painkiller.

"I sat down and cried like a baby," Webb says. His left hand has moved to his face now, cupping one lens of his geometric wire-rims. The pale blue eyes are squinting. He is very much in control. "I mean, the kid's only 18 years old and here's his arm off. What do you do? Damn, if Mack didn't look up at me and say 'Hey, Lieutenant, knock off that s - . It's only an arm.'"

At this, James Webb erupts quick, loud stutters of laughter.

And Mack Himself?

"Oh, the Skipper?" he said when contacted by phone the other day to ask about his former C.O. It was 5 o'clock in Tennessee adn Mack was done for the day. He loves his job, he said. "I'm kinda like a pig in a big Southern mudhole." He talked with a round, soft accent.

"Webb was the kind of guy you'd go out on a patrol with unarmed if he told you to. I carried his radio, slept next to the dude. He was never you spit-and-polish type. If a bird colonel came along, the Skipper would sure hell get out there and talk to him. But you knew he'd rather be in the hootch playing Tonk with the guys. his loyalty went down, not up."

He was asked about "Fields of Fire." "Let me put it this way: I ain't cried in years till I read that book - and I'm not finished yet. There are some things in there you want to remember and some things you don't ever want to remember."

Mike McGarvey has a tattoo at the base of his stump. It says, "Cut Along Dotted Line."

James Webb and Mack got together last year. It had been eight years. Mack says they cuffed and cried and afterward Webb told him get a haircut. Making a Mark In Life

James Webb was raised military - on bases around the country and overseas. The Webbs are from Arkansas and Eastern Kentucky, mostly. (Webb's accent is deep and broad.) Webb's grandfather was a sharecropper who never quite realized his dreams. His name was Robert E. Lee Webb. Webb's great-great-grandfather served in the Civil War. "He was a 43-year-old private in the Virginia Cavalry," he grins. Webb's grandmother used to tell him stories about the great tradition of the Webbs. There is a character like her in the novel.

Webb grew up boxing - on base teams, in high school, at Annapolis. You can tell by looking at his nose. There is something cat-wary about his stance, something graceful about his smaller moves, even if his walk is big and loose and semiflatfooted. (There is also a slight gimp in it.) The grace and preclsion show up in other ways, too - the way he removes a staple from a sheaf of papers, or aligns a sheet perfectly along the margins of a copy machine.

At Annapolis, he really couldn't get serious about boxing. "The commandant was scared of the sport and made us wear these big balloon gloves. I was a jabber and counterpuncher." He did have one good fight, though - in the summer of '67 on the hangar deck of an aircraft carrier anchored in the Mediterranean. He took on the Navy champion of the Pacific Northwest. "The guy was undefeated in 29 bouts. He knocked me down, but I broke his nose and won the fight." The fight was televised through the task force.

These days Webb can't play stress sports. The arthritic softening in his knee is too bad. "They told me it's like Teflon coating in a pan that has begun to curl and come off in your fingers," he says, laughing in a kind of odd, crinkly way to cover the awkwardness. "When it gets old, it just goes." He knows it's probably not good for him but he runs a couple of miles per week. Mostly, he just works out on his climbing rope (sometimes twice a day), which he keeps in a neighbor's yard. He looks boot-camp ready.

Webb was hit twice in Vietnam, the first time with a B-40, which is a shoulder-fired, rocket-propelled grenade launcher used against tanks and people, the second time with a grenade. The grenade blew him 10 feet in the air into water. Altogether, he took metal in 13 places. He still carries a piece in the back of his head. "I don't like talking about them . . . the scars," he says. "Compared to some I got away lucky." He brightens. "Hey, I got a buddy who's got a piece of metal right in the middle of his heart. The doctors told him just not to get in a car accident." Again, a laugh.

"The bravest men I knew over there are all dead," he says flatly. "They lost regard for their lives. Somehow, their need to react became more important than anything else. It's tied up with respect and self-respect, not losing inertia. Of course, I also knew people who were comfortable being ineffective. For them, self-preservation was more important than dignity."

Growing up in the Webb family, there was lots of regard for reason, for maintaining dispassion. There was also much talk of goal, of achievement, of making your mark in life. "It's funny, but loyalty was always a big discussion in our family. At the dinner table we'd talk about what would work and what wouldn't work in managing people." (Webb's younger brother, Gary, was also a Marine, also went to Vietnam.) Give and Take

Twenty-six days after he mustered from the Corps, James Webb entered Georgetown Law School. "Well, it was time to get things going," he says. That first year, trying to catch all that had flowed on, like a river, while he was gone, was tough on him, he says. "I gave a lot of ground." He gained it back though.

He knows who he is as a civilian now - few people in the world can change his mind once he's made it up. "There are lines to draw. There are certain things not debatable." At Georgetown, he earned a nickname for a time - "The Reasonable and Honest Fascist." His major flaw, he says, is being "extremely compulsive." But James Webb doesn't like talking about faults, even if that one cuts both ways.

He hardly ever pulls a punch. Asked if he has seen the Vietnam film, "Coming Home," starring Jane Fonda, he says, looking you cold in the eye knowing it will be printed: "Jane Fonda can kiss my ass. I wouldn't pay to see her cut her wrists. Although I might."

Webb's wife is also a lawyer. They met in high school and got married after his graduation from Annapolis. They went to law school together, one during the day, the other at night. They have an 8-year-old daughter, Amy. Webb wonders what Amy will think of the book, its hard language and violence. "I've never told her anything about Vietnam. Except she's seen my friends. Guys come in without arms or legs. So-and-so has to sit down all the time. She must know."

Webb's father has read the book. He was moved, says his son. "He told me he hoped Vietnam would open up for travel and that I would take him there." Advance copies were also sent to some old-guard Marines. One general said it was the "Battle Cry" of the Vietnam war; another said it stunk, distorted what the Marines really are. Webb says he's loyal to the Corps.

He wrote the book over 18 months, beginning after he left law school. He often worked seven days a week, 14 hours a day. After lunch, he would take breaks and read British poetry. Funny, he never liked art or art museums until he started writing. Now he's crazy for Impressionism. he knows what Hemingway meant when he said he learned to write [WORD ILLEGIBLE]from looking at Cezanno.

James Webb says he isn't bitter. I am not personally bitter in the material sense think I'm spiritually bitter. I can't even articulate the idea. You look around and see the people your own age who are getting along best. They're either the guy who managed to avoid the war altogether, with no stigma, or the guy who was an activist - and has now converted that into political or othe credentials. No Right or Wrong

Webb appears to be getting along professionally as assistant minority counsel to the House Veterans Committee. He has been in his job 18 months. Though he is the only Vietnam vet on the committee (the other members go back to World War II), he thinks ground is being gained. He would like it to go faster.

Draft evaders still rankle. "For every guy who went to Canada, four other guys died in Vietnam. I have no problem with a guy making an act of conscience - if it was really that. But look, more Marines died in Vietnam than in all of World War II. Think of it: Iwo Jima, Tarawa." He doesn't finish.

He says the way he feels about Vietnam is "almost fragile. I'm very protective about it. I'm not going to say it was moral or immoral. It's ambiguous. I don't think it's important right now to know whether that war was right or wrong. History will decide. The important thing is to understand what those guys went through and allow them their period of working it out."

So James Webb, the unlikely novelist, partly to work it out, has written a blunt, bloody novel (with some striking poetic passages), where a man's leg "gurgled like a stopped-up drain," where a hand with a wedding ring on it oddly rests, in perfect shape, outside a tent. Prentice-Hall, Webb's publisher, has arranged a nationwide tour starting next week. The publicists call it "the most powerful war novel in a generation."

It may not be that, but it may make Webb a writer of sudden rank, one with a newfound financial liberty. On the sidewalk the other day, standing in the fierce 3 o'clock sun, looking somehow five or six years younger than the man in government-issue clothing on the back of his book's dust jacket, James Webb considered this. Suddenly he grinned.

"Ah," he said. "But liberty is just the luxury of self-discipline."