THE ASSASSINATION of longtime dictator Park Chung Hee last October revived prospects for a staged return to democratic rule in South Korea. The effort was set back in December when a group of disloyal generals ousted the military caretaker regime presiding over the transition. Still, a civilian government was left in place to work on drafting a new constitution, and the political life of the country continued to unfold cautiously. Over the weekend, however, that fragile effort was completely wiped out. The same group of power-hungry generals, using the recent student demonstrations as a pretext, imposed martial law and went on to suppress the students, to arrest key opposition figures and, yesterday, to hustle the civilian cabinet off the stage. Except for the fact that there is no sign of an impending North Korean invasion, things could not be worse.

Naturally, the coup-makers say an "emergency situation" had been produced by North Korean "troop movements" and by civil disturbances. But of course there is no supporting evidence. The more likely explanation lies in the situation being created by South Korea's tentative movements toward a new constitution to replace the one that had been the instrument of Park Chung Hee's personal rule. The civilians were under pressure from the generals to write a restrictive draft even while they were being urged by students and others, including representatives of the country's important middle class, to open up the political system. Either the recent student demonstrations frightened some of the generals or they simply took advantage of them to make their move. In any event, they have now assured South Korea an indefinite period of instability, since the constitution they prefer is bound to reflect their own craving for dominance and not the political interests of the broader society.

For the United States, the new turn is a bitter disappointment, the more so for the common awareness that the generals obviously chose a moment of the Carter administration's acute discomfort elsewhere in order to ensure that the administration would not react stronly. If the generals think that they have pulled a fast one on Jimmy Carter, however, they will still have to deal with the fact that they have profoundly compromised their own long-haul position in the United States. No foe could have struck a harder blow at the basis of South Korea's standing and security than the generals who now monopolize power.