As any devout environmentalist can testify, Interior Secretary James Watt is not much given to leaving well enough alone. But what he does to wilderness, clean air and clear waters is at least well within an interior secretary's proper sphere of influence. Only up to a point can the same be said of what Watt is now doing to undo all the hard work and painful compromise that went into the creation of the Vietnam War Memorial.

Perhaps you thought the issue had been put to rest. From the time a lone former infantryman, Jan Scruggs, first walked in unannounced to broach the idea to Sen. Charles Mathias in 1979, the pace was swift. Mathias soon had the full Senate as cosponsors of a resolution that also swept through the House.

It set aside two acres in Constitution Gardens (a piece of parkland under Interior's custody) as the memorial site. The Vietnam Veterans' Memorial Fund raised the necessary construction money from private sources. An open competition attracted 1,400 would-be designers.

The winner was a young Chinese- American architectural student, Maya Ying Lin. Her design was provocative and instantly controversial: a sunken V-shaped wall of black marble, bearing the names of all the war's 57,000-plus dead and missing. Critics saw it as a "black gash," memorializing the dead without honoring those who served and survived--an anti-war statement. Before he saw it, Watt proclaimed it "an act of treason."

The rest of the story is tangled and controversial. Suffice it to say that after many special panels and committees and much impassioned argument, a bargain was struck. To Maya Lin's memorial would be added a conventional, heroic sculpture of three fighting men and an American flag.

But the infighting did not end there. It came down to a question of how to integrate all three elements in a way that would somehow reconcile the reasonable and profound ambivalence with which Americans continue to read the lessons and the meaning of Vietnam--without compromising the artistic integrity of the winning design.

By last October, you would have been justified in thinking that a solution was at hand. The Commission of Fine Arts, whose final approval is required by law, unanimously accepted in principle the addition of the flagpole and the sculpture. But it suggested that they be placed a short distance away from the open end of the memorial to "help enhance the entrance experience." Watt and a vociferous group of conservative critics, including a few strong voices in Congress, wanted them at the point of the V as an integral, symmetrical appendage.

In the course of a moving Salute to Vietnam Veterans' Week in November, you would have thought the issue was, well, incidental. Watt allowed the dedication of the monument to go forward, while reserving his right to stick by his own view of how the memorial should be laid out. But the nice aesthetic distinctions between his view and that of the Fine Arts Commission seemed lost in the outpouring of warmth and long overdue thanksgiving that found its expression in that November week.

If Watt does what he now says he intends to do, however, the rift over the monument's design may well widen-- and in a way that can only rub raw the deeper Vietnam wounds. For Watt seems determined to set himself up as the final arbiter of what--aesthetically, artistically, symbolically--would constitute a proper memorial statement on the Vietnam War.

To recent visitors, he makes no bones about it: the issue is political. Under right-wing political pressure, he intends to propose to the next Fine Arts Commission meeting precisely the design that the commission seems determined to disapprove.

A compromise is still possible. Conciliatory forces are working on Watt to discharge his obligations by giving the commission freedom to choose among clearly defined options, including a design that the commission would be likely to approve. If this fails, the closing days of the lame-duck session offer a foretaste of things to come. Only by the intervention of Sen. Mathias was a House resolution headed off in the Senate that would have legislated the layout favored by the so-called "militarist" conservatives. One of its promoters, Rep. Don Bailey, was himself a lame duck. But its cosponsor in the House, Rep. Duncan Hunter, is back, and likely to renew the effort.

One result could be congressional hearings--a sure way to rekindle the war over the Vietnam War in a mindless and impassioned way. Ultimately, Watt may force the Reagan administration and perhaps even the president himself into taking a position in an argument that would add nothing to the healing of wounds and still less to any useful effort to learn from the Vietnam experience.