When a new creation by Robert Wilson gets stag -- as little known as he may be to the average run of American theatergoers -- it's inevitably an Event with a big "E." This was certainly the case with the portion of his "the CIVIL warS: a tree is best measured when it's down" mounted by the American Repertory Theatre at Harvard's 500-seat Loeb Drama Center, and seen in its "national press opening" in Cambridge this past weekend though it has been in previews for more than a week.

Like most of Wilson's work -- though not, perhaps, to the same epoch-making degree as his opera "Einstein on the Beach," in collaboration with composer Philip Glass -- "the CIVIL warS" segment at Loeb is radically unconventional, startling, hypnotic and provocative. Despite some less than trenchant aspects, it once again shows us Wilson stretching the limits of artistic possibility with an imaginative daring that has few parallels on the contemporary scene.

The sense of occasion was magnified by the complicated genesis and production history of "the CIVIL warS," and the thorny path that led Wilson to the present ART production (which runs through March 17). As originally conceived, "the CIVIL warS" -- which occupied Wilson and his multinational collaborators for six years -- was to be a 12-hour opus in five acts, 15 scenes and 13 "knee plays" or interludes. Parts of the work have had separate premieres over the last two years in Rotterdam, Cologne, Rome and Minneapolis (the "knee plays"), and others have received workshop performances in Tokyo and Marseilles. But the projected "complete" staging for the Olympic Arts Festival in Los Angeles last year fell through for lack of funds, and until now only an abortive "concert performance" of one act has taken place in this country.

Hence, the ART production of Act III, Scene E, and Act IV, Scene A and Epilogue, is the first American presentation of a major slice of the work. It's also the first time Wilson has worked with an established company of American actors. One of the things the event demonstrates is that Wilson hasn't left the realm of the controversial. The Loeb production runs three hours -- not long for Wilson, who's made one piece that took seven days to enact -- and has an intermission, a Wilson rarity. Yet, here was a performance in Cambridge, amid the greatest concentration of intelligentsia in the nation, and at least a third of Saturday night's audience walked out, presumably in indignation or boredom or both. At one point, it was cause for general merriment -- a group of people got up to leave just as one of the actors spoke the line "Oh, stay by me and go you not!"

Though the inspirational starting point was the historic American conflict, "the CIVIL warS" is not just about the struggle between North and South, but more generally, about divisive fissures within individual men, nations and humanity as a whole. This is the reason, Wilson explains, for the peculiar capitalization of the title: "I think of it as a civil struggle that's gone on through all times . . . any number of historical confrontations, not necessarily violent, which comment on man's long journey towards brotherhood."

Thus Act III, Scene E, opens with a Civil War battlefield scene that might have come right out of Mathew Brady's tintypes, and the concluding Act IV Epilogue introduces the figure of Lincoln -- a woman on stilts raising her to 20-foot height, complete with beard and stovepipe hat. But in between, the imagery ranges from an itinerant, stylized family group to weird, unclassifiable apparitions to the recurrent personage of Frederick the Great. Frederick becomes, among other things, a symbol of the schizophrenic soul of the German nation, the Prussian monarch having been at once a purveyor of enlightenment and a brutal despot.

A description of the contents of the production is apt to sound, in print, like a chaotic jumble of unrelated fantasies. The haunting opening scene, with its tents, soldiers, muskets, campfire, mists, crickets and overlapped texts coming from multiple speakers, yields to others, which display the family seated around a table; a child building a Stonehenge out of toy bricks; a space missile traveling across the upper reaches of the stage and then descending to almost incinerate an older woman; a dance for two polar bears; Frederick on his deathbed, and later, shooting a dog with a pistol; a bizarre rendition of Schubert's "Erlko nig" by Frederick's mother as she munches bonbons and feeds her lap dog; and the Epilogue, in which Lincoln is joined by a screeching Snow Owl, an Earth Mother and a tattered King Lear cradling crumpled newspapers in his arms and bemoaning his dead Cordelia.

All this is folded within a multimedia cocoon -- the work of Wilson himself and such collaborators as East German playwright Heiner Mu ller, set designer Tom Kamm, costumer Yoshio Yabara, lighting designer Jennifer Tipton, composers Hans Peter Kuhn, Michael Galasso, Philip Glass and others, and a battery of filmmakers and projectionists. The multiple role portrayals by the ART cast of eight principal actors and an ensemble of 18 are surrounded with layers of verbal text, sound, music, scrims, ramps, traps, visuals and props that make for a continuous, characteristically slow-moving and repetitive sensory web.

There are lapses and saggings, but mostly it's the genius of Wilson to orchestrate these elements into an integrated experience that, while only dimly explicable in rational terms, generates tremendous wonder, surprise and feeling. It's as if Wilson's eye burrows deeply into our unconscious store of memory and emotion, and uncovers astonishing connections that lurk there waiting to be switched on. It's the method of collage, heightened to an extraordinary revelatory power by an artist with a nearly infallible instinct for the reconciliation of opposites. As Robert Brustein, artistic director of ART, recently put it, Wilson "launches the theater into new dimensions of the unknown, propelling our imaginations into the expanding universe of art."