THE MOST CURIOUS ASPECT of the disintegration that has been visibly under way in Iran in the last few weeks is that the shah - a dictator, after all - has not been using the full power available to him to take the situation in hand. The spectacle of troops standing by while the mob rampaged remains as a kind of snapshot of the whole turbulent scene. Some people have thought the shah might, cleverly, be letting his opposition expose and discredit itself. Others have thought he might have suffered a lapse of judgment or even nerve.

Whatever the explanation, the shah has now made his move - dismissing parliament and appointing a military government with instructions to restore order. It seems to us about the only thing he could have done at this time. The considerable concessions he made in recent weeks had been hurled back in his face. The Moslem zealot who runs the principal opposition movement from exile in France had responded to the political openings offered to him by refusing to compromise and urging his supporters (and the army) to continue struggling to bring down the shah. The leftist National Front had turned its back, too. The economy was a shambles, and the country was exploding. One does not have to issue a blanket endorsement of either the shah's past rule or his current policy to accept that, if he were going to try to keep governing, he had to change course and take the immense risk of trying to reassert his power.

The shah, in turning openly to his generals, pledged that "after order is restored" he will move on to form a national unity government and hold free elections. That is a tall order for a country virtually overcome by chaos and lacking a strong democratic tradition, but it was a necessary and useful pledge on at least two counts.First, the tensions that have exploded in Iran can finally be contained, if at all, only if there is a suitable channel for their political expression. That will inevitably require a substantial diminution of the autocratic power wielded by the shah in the past. Second, the shah needed to make such a pledge to be sure of gaining full American backing, which is crucial to his survival.

The Carter administration seems to have been of several minds about the troubles in Tehran. There has been a strong tendency to give conspicuous moral and political support to the shah, long regarded as a major strategic and economic asset for the United States. At the same time, there has been a lingering tendency to ask if the United States ought to step back a pace from a regime that is not only flawed but uncertain of lasting out the storm. The administration's decision, made evident this week, was to stick strongly with the shah. Anything else would have been a mortal blow against him and a truly damaging signal of American inconstancy. But the administration hitched its support to an explicit expectation that Iran will move "as rapidly as possible" to elected civilian rule.

On the shah rests the principal responsibility for preventing Iran from descending into the sort of utter chaos that would all but wipe out the major gains he has achieved in national welfare and regional stability. The countdown has begun.