CHILE'S JUNTA is ending some of the harshest aspects of its rule and moving back cautiously toward a form of modified constitutionalism. Its sudden burst of cooperation with the U.S. inquiry into the Washington murders of anti-junta figure Orlando Letelier and a collegaue is particularly noteworthy. Given the notoriety of the regime and the ubiquity of rightist dictatorships, it is worth exploring what's behind the change.

Factor I would have to be the junta's success in consolidating its power. That has meant a brutal repression of its foes, reshaping of the economy to favor people of property and their creditors, and a restoration of public tranquillity. The military leadership now finds it politically feasible, and internationally useful, to allow the Chilean people's apparently irrepressible political spirit to start being expressed again. A kind of lively and increasingly obvious politics is going on inside the junta. it has worked for the cause of relaxation so far.

Factor 2 would be the particular combination of censure and tolerance that Chile has encountered on the international scene. Some part of the censure, the part coming from leftist authoritarian regimes, may have been dismissible as hypocrisy and propaganda. But fair-minded, nonideological criticism from traditional friends surely has had an effect. We wonder, however, whether the various forms of pressure would have worked if Chile did not also have available certain sources of outside help, especially private banks. Some opponents of the junta complain that Chile's access to private American credit has undercut the United States' Carter-period policy of denying the Pinochet regime public loans. But the private money has helped provide the stability underlying recent liberalization.

The United States, which had a role in bringing on Chile's coup in 1973, has since had a role in steering the country back in the other direction. That is as it should be. Nothing can undo the human and political devastation to which this country contributed, but it would be intolerable if the United States were still offically supporting the makers of the coup. At the same time, it is necessary to concede that putting Humpty Dumpty back together is not a simple task. It takes more than good intent. it take wise policy - and some luck. Since the junta still has a long way to go, it seems sensible to stay with the same pressures, and openings, that are working now.