By GEORGE JOHNSON

UNTIL very recently, the most perplexing mystery of Southwestern archeology -- what caused the collapse of the ancient empire of the Anasazi -- seemed all but solved. Careful scrutiny of tree-ring records seemed to establish that in the late 1200's a prolonged dry spell called the Great Drought drove these people, the ancestors of today's pueblo Indians, to abandon their magnificent stone villages at Mesa Verde and elsewhere on the Colorado Plateau, never to return again.

But in the last few years, Southwestern archeology has been shaken with a quiet revolution. Textbooks are being rewritten as the common wisdom, taught to generations of students, is overturned. ''Nobody is talking about great droughts anymore,'' said Dr. Linda Cordell, a professor of anthropology at the University of Colorado in Boulder and director of the natural history museum there. ''The mystery of the Anasazi is an open book again.''

Groundbreaking climatological studies have convinced many archeologists that the ''so-called Great Drought,'' as detractors now call it, simply was not bad enough to be the deciding factor in the sudden evacuation, in which tens of thousands of Anasazi (the name, pronounced an-a-SAH-zee, means ''enemy ancestors'' in Navajo) moved to the Hopi mesas in northeastern Arizona, to the Zuni lands in western New Mexico and to dozens of adobe villages in the watershed of the Rio Grande.

''There are just too many little discrepancies,'' said Dr. Eric Blinman of the Office of Archeological Studies of the Museum of New Mexico in Santa Fe. Recent studies have shown, for example, that the evacuation actually began before the dry spell set in. Even more telling is evidence that the Anasazi had weathered many severe droughts in the past. Why did the one in the late 13th century cause an entire population to abandon the settlements they had worked so hard to build?

''The Great Drought may have been the last straw,'' said Dr. John Ware, another archeologist at the Museum of New Mexico. ''But in and of itself, it just wasn't enough.''

As they sift the evidence, archeologists are finding surprising new elements that may have conspired with drier weather to bring about the calamitous fall. Belying the popular image of the Anasazi as a peaceable kingdom of farmers and potters, some of the new research puts the blame for the collapse on a bloody internecine war. Other researchers are trying to combine archeological evidence with anthropological studies of the modern pueblo Indians to make the case that the Anasazi were roiled by a religious crisis as divisive as European medieval heresies. In some scenarios, the Anasazi were pulled farther south en masse by an attractive new religion.

Trying to recreate ideology from artifacts requires huge stretches of the imagination, but archeologists find it telling that many of the Anasazi religious structures -- like the tall cylindrical tower kivas found at Hovenweep in southeastern Utah -- were not re-established in the new homelands. Once the Anasazi left the old empire, it seems, the ideological slate was wiped clean.

Many archeologists long complained that the Great Drought story seemed a little too pat. The recent flowering of theories began in 1990 when an archeology student at Washington State University, Carla Van West, startled a conference at Crow Canyon Archeological Center in Cortez, Colo., with research undermining the great drought theory.

Drawing on records compiled by the Laboratory of Tree-Ring Research at the University of Arizona, Dr. Van West calculated just how much moisture a 700-square-mile area near Mesa Verde in southern Colorado had actually received before abandonment. Correlating these data with information on productivity of various soil types, modern crop yields, and detailed geography, she concluded that enough corn could have been grown during the drought to support the population.

''What her work does is to show that there is not good evidence for a drought so profound that it literally wiped out all farming in the area,'' said Dr. William Lipe, an archeologist at Washington State University. ''That simple version just won't hold up anymore.''

However, from studying human bones left in the ruins, archeologists were pretty sure that the Anasazi had been suffering from malnutrition, shorter life spans and increased infant mortality. If there was the potential to grow enough food, then why were people starving?

Dr. Ware was one of the skeptics. ''I felt like a coroner in the local morgue,'' he said. ''The investigating officer comes in and tells me the accident wasn't that bad. But I'm seeing the destruction all around me.''

Dr. Van West's study helped set off a search for new, more complex explanations for the collapse. An assumption built into Dr. Van West's model was that the Anasazi could simply move to new plots nearby whenever the land they were working became too dry. Some climatological evidence, based on tree-ring and pollen studies, suggests that Anasazi farmers may have been kept from moving to higher, moister grounds by a worldwide cooling trend called the Little Age Ice. According to this theory, the Anasazi were squeezed from two directions: lower elevations were too dry for farming, higher ones too cold.

Dr. Michael Adler, an archeologist at Southern Methodist University, argues that the Anasazi were not able to move around freely because their once open range was becoming balkanized into hostile fiefdoms. Archeological evidence shows that in this period, perhaps as a reaction to drier weather, people in the Mesa Verde area began building dams and canals to trap and divert water to terraced fields. They were ''investing in landscapes,'' Dr. Adler said, and presumably began to feel more territorial about where they had settled. ''The land was filling up with claims and rights,'' he said. ''People had to ask before they used.''

The result may have been conflict and warfare. Near Kayenta in northeastern Arizona, Dr. Jonathan Haas of the Field Museum in Chicago has been studying a group of Anasazi villages that relocated from the canyons to the high mesa tops around 1250 A.D. The only reason Dr. Haas can see for a move so far from water and arable land is defense against enemies. ''If you don't have enough food to feed your children, you go raiding,'' he said. ''And once I raid you, then you have justification to raid back -- the revenge motive. And so warfare becomes endemic in the 13th century.''

The settlements Dr. Haas is studying near Kayenta seem to have been carefully arranged so that each village could watch out for its neighbors. In one case, a notch was cut into a ridge to create a line-of-sight view. Dr. Haas theorizes that interdependent networks like this would have been as fragile as a house of cards. ''If one community drops out and it is strategically located, then the others become unviable,'' he said. ''There is a ripple effect. ''

But other archeologists have trouble envisioning how even drought, balkanization and warfare could make an entire civilization evacuate. Why didn't the winners of the Anasazi wars stay and enjoy the spoils?

''The peculiar character of the abandonment is its completeness, its rapidity,'' said Dr. Lipe of Washington State. ''This suggests that some kind of 'pull' was operating as well -- or an ideology favoring migration.''

Analyzing the spread of religious symbols found on rocks or pottery and the distribution of ceremonial structures, some archeologists argue that the Anasazi may have been pulled from their homeland by a new religion emerging among neighbors to the south.

One candidate is the Kachina religion with its masks, familiar to visitors to Zuni and Hopi pueblos. Unlike many of the secret organizations in the modern pueblos, the Kachina societies, in which spirits of dead ancestors act as intermediaries to the gods, are open to everyone. Some archeologists have surmised that this egalitarian spirit would have had great appeal to a civilization, like the Anasazi's, that was entering a dark age. ''There was hot stuff going on down south,'' said Dr. Steve Lekson, a research associate at the University of Colorado Museum. ''There was a new, vibrant, flashy, more democratic ideology.''

But archeologists disagree on whether the archeological record of Kachina-like icons and other artifacts puts the religion on the scene early enough to have attracted Anasazi. And even if there were some compelling new religion, these skeptics argue, why wouldn't the ideas just spread northward? People did not have to flock to Rome to embrace Christianity.

Although he does not buy the notion of a ''pull'' exerted by the Kachina religion or some other faith, Dr. Ware believes the Anasazi world was indeed rocked by a spiritual crisis catastrophic enough to cause a collapse of a civilization. One major clue is the lack of traditional Anasazi ceremonial structures -- like the tower kivas -- in the Rio Grande pueblos and among the Zuni and Hopi. Once the uprooted Anasazi arrived in their new homes, they apparently embraced a variety of new beliefs.

Anthropologists studying 20th-century pueblos have found a bewildering mix of secret societies co-existing with the more recent Kachina religion. There are hunt societies, medicine societies and societies of sacred clowns. In addition, pueblos are often divided into two factions, called the summer and winter or the squash and turquoise people.

Anthropologists are fairly sure that these new organizations were not imported by the Anasazi but sprang up after their arrival. That, they say, is why the Kachina religion is strong in the western pueblos of Hopi and Zuni, but fades eastward. On the other hand, the division into summer and winter people is strongest in the east, fading to the west.

''There was a major reshuffling of organizational systems once the Anasazi got here,'' Dr. Ware said. ''That suggests there was a catastrophe.''

It may have been a change in climate after all, but one different from the drought Dr. Van West questions. Recent climatological studies by other scientists suggest that rainfall patterns were disrupted in a way that might have made the Anasazi disillusioned with their old religion. Studying tree rings from 27 sites across the Southwest, Dr. Jeffrey Dean of the University of the Arizona tree-ring laboratory has found evidence of a major disruption in the area's typical rainfall. Suddenly, the customary pattern of heavy snows in the winter followed by summer monsoons had become unpredictable. Even if there was not a great drought, moisture may have been coming at the wrong times. The summer rains, so necessary to keep the spring crops from dying, were no longer reliable. The rain dances were not working anymore.

''This would have represented a major upset,'' Dr. Dean said. ''And it happens to occur exactly at the time when you're getting all these population movements and cultural changes.''

What all these theories have in common is a rejection of the old notion of the environment as the single determining cause, with the Anasazi no more than passive pawns of blind forces. ''You have to look further than the environment,'' said Dr. David R. Wilcox, senior research archeologist at the Museum of Northern Arizona in Flagstaff. ''There is a whole social dimension to this process of abandonment that we are only now starting to grapple with.''