SO FINALLY President Park Chung Hee of South Korea lost power -- and his life -- as he had held it for 18 difficult years: by force. Somehow, he was not shot while having dinner with the director of the Korean itelligence agency, one of the chief props of his rule. He had survived, or so it seemed, the latest wave of popular unrest, emerging, if anything, even stronger after the parlimentary oppostion's quixotic protest of quitting its seats. A chastened Carter administration, easing its earlier human rights emphasis, had just declared it would let nothing get in the way of security considerations in future dealings with Seoul. Mr. Park looked set.

:Mr. Park was not a popular man in this country. His granite features, a kind of steroetypical military manner and his authoritarian political style encouraged the suspicion that he exploited fears of communist North Korea's hostility to win American tolerance for his own repressive rule. Yet he won a following at home by making South Korea one of the undoubted economic successes of the developing world. Even his bitterest foes agreed, to their own political detriment, that North Korea's consummate hostility did in fact make a vigilant anti-communism Seoul's only feasible stand.

:It would be nice to think that President Park's death creates the first real opening in 18 years for the political opposition. Unfortunately, the fact that his rule ended not as the climax to a political protest but rather in a gangland-stlye scene suggests that a harsh insider's power struggle for the immediate succession may take place and that the winner of it will lack an evident popular mandate.

:Bet here the United States may conceivably play a helpful role. Its first reactions -- to put American troops on alert and to warn North Korea not to exploit the confusion -- were meant to keep things calm. As Koreans start to untangle a political system and constitution designed for the arbitrary requirements of one man, Mr. Park, Washington should look for effective ways, necessarily modest ones, to move the politics of its longtime ally in the direction of openess. Later on, with a new government's cooperation, it amy be possible to explore ways to ease the tensions that have long riven the Korean peninsula.

:It is silly to say that it was only President Park who kept South Korea from fulfilling the democratic promise the United States has held out for it. It would be equally wrong to say that a fair measure of representative government is incompatible with the culture, and the security situation, of Seoul.