WHEN EMMYLOU Harris shares the stage at DAR Constitution Hall on Friday with Willie Nelson, Sheryl Crow and Lucinda Williams, it will benefit the Washington-based Campaign for a Landmine Free World. It's also the result of a two-year crusade growing out of a friendship that first bloomed in the early '70s in such Washington clubs as Clyde's, the Childe Harold and Tammany Hall.

That particular circuit is where the Alabama-born, Virginia-bred Harris first began making a namefor herself before establishing herself as one of the crucial voices in country music. It's also where she first met Gail Griffiths, who would become associate director of the Vietnam Veterans of America Foundation, which launched the Campaign for a Landmine Free World.

During one of their frequent phone conversations, Harris and Griffiths shared a serendipitous experience.

"I had just read an article in the New Yorker about land mines," says Harris, recounting initial reports about the 110 million land mines worldwide that kill or horribly maim 26,000 people annually, most of them civilians, many of them children.

"Like most people, I was shocked at the sheer magnitude of the number of land mines in the ground," Harris recalls. "I thought it might have been a misprint or a mistake, except you know how the New Yorker is about fact-checking."

As it happened, Harris had just finished reading Michael Ondaatje's "The English Patient," which addresses the disarming of land mines, and both she and Griffiths had recently seen the 1996 Oscar-winning film adaptation. Soon after, Harris met with Bobby Muller, director of the veterans' foundation, and hosted a Nashville gathering to get other country artists involved in the land mine issue.

According to Harris, "you don't have to take a political stance, because it is a militarily irresponsible weapon that's impacting civilians around the world, most often in very poor countries." Harris, who has lobbied on Capitol Hill, first championed the cause during daily news conferences at last year's Lilith Fair, encouraging concertgoers to send pre-printed postcards urging President Clinton to sign the international treaty to ban land mines. Clinton has committed the United States to signing the treaty in 2006; so far, 125 countries have signed the treaty.

Two years ago, media coverage of the land mine issue was minimal -- until Princess Diana adopted it as a personal cause just months before her death. Soon after, the 1997 Nobel Peace Prize was awarded to the International Campaign to Ban Landmines and its coordinator, Jody Williams.

Now, Harris explains, Muller and the veterans' foundation are focusing on an international effort to identify, remove and destroy the tens of millions of mines that remain in the ground -- and could remain there for as long as a century. "It's not enough to say we won't put any more in the ground; we've got to get them out," says Harris. "It comes down to basic courtesy, something you learn in kindergarten: You clean up after your mess when you're through. Until we do, these countries are going to be hostages. They're going to continue to live the war many, many years after peace has been declared."

Last fall, Harris witnessed the results of land mine terror when she, Muller and Griffiths traveled to Vietnam and Cambodia; in the latter country, it's estimated that 60,000 civilians have been killed or maimed since 1970 as a result of as many as 10 million land mines laid down over 30 years by American, Vietnamese, Cambodian and Khmer Rouge armies.

In Phnom Penh, the contingent visited the VVAF-sponsored Kien Khleang Rehabilitation Center, the largest prosthetics clinic in Cambodia (another clinic supported by the foundation now operates in Angola). Proceeds from Friday's concert will help support such rehabilitation services, as well as the international effort to remove mines from the ground.

On the musical front, Harris recently released "Spyboy," a live album recorded over the last two years with her band, Spyboy.

"I knew we were a good band, but you always worry about whether you capture that on tape, and I felt that we did," says Harris, whose career was honored last year with the three-CD boxed set, "Diamonds in the Rough." Her last studio album was 1995's Grammy-winning, Daniel Lanois-produced "Wrecking Ball," and Harris notes that since she was on the road promoting that album for much of the last three years, there was no time to work on a follow-up.

"It's hard for me not to have product out there," she says. "Just psychologically, it makes me feel like I'm sloughing off, though I know I'm not. So Spyboy' is my note to the teacher: This is what I've been doing,' " Harris adds with a laugh.

It's also a one-time project out on her former manager's label, representing what Harris describes as "taking a sabbatical from being responsible. I left my record company and my manager, and I let my band go, intending to let the field lie fallow, thinking I would just sit around writing for a year."

Though she's not currently signed to a label, Harris seems busier than ever. She's prominently featured on concert-mate Willie Nelson's new album, "Teatro"; it, too, was produced by Lanois and features Lanois's masterful song, "The Maker," which Harris first recorded on "Wrecking Ball."

Harris is also executive producing a tribute to her original mentor, Gram Parsons, and Asylum Records is scheduled to release "I Feel the Blues Movin' In," the long-recorded, long-contractually-delayed follow-up to "Trio." That glorious collaboration with Dolly Parton and Linda Ronstadt was the only country album to reach the pop Top 10 in the '80s.

"I'm taking a very Zen approach to it," says Harris. "It's a lovely record, and I hope it comes out."

In the meantime, she's been in Tuscon, recording a new album with her old pal Ronstadt. Perhaps it might be called "Duo?"

"It is just the two of us," Harris laughs. "It's something we've talked about for a long, long time. It just felt like a really good thing to do, and so far it's been a real joy.

"We both love singing harmony and we like the same kinds of songs, songs that are kind of hard to categorize," Harris adds. "And we are interpreters, for the most part -- there's not too many of us around. Most everybody is a singer-songwriter, so it's nice to be in the company of somebody who really gets off on singing somebody else's songs!"

And Friday night, she'll also get a chance to visit with neighbor Lucinda Williams. "We live one house apart in Nashville but we haven't seen each other in so long," Harris complains, "that we have to come to Washington to do a show in order to see each other!" EMMYLOU HARRIS -- Appearing Friday at DAR Constitution Hall with Willie Nelson, Sheryl Crow, Steve Earle and Lucinda Williams in a benefit concert for the Campaign for a Landmine Free World. * To hear a free Sound Bite from "Spyboy," call Post-Haste at 202/334-9000 and press 8130. (Prince William residents, call 690-4110.) CAPTION: Emmylou Harris and Willie Nelson (pictured with producer Daniel Lanois, center) will perform at a benefit concert. ec