LAST week, when South Korea celebrated its Liberation Day, clashes between students and police in Seoul flashed briefly across US television screens. Such scenes seem to be a hardy perennial of TV news. The whys and wherefores of the protests are seldom explained. But if no good pictures have come into the newsroom from elsewhere, why not show 30 seconds of Korean teargas and firebombs?The real story of South Korea today is the successful transition that this country of 45 million has made from authoritarianism to democracy during the past four years, in contrast to the communist dictatorship that prevails in North Korea. You can find plenty of complaints about President Roh Tae Woo's low-key performance. Under his administration, however, Seoul has moved away from the military-bureaucratic state it used to be, to one in which political parties compete at both the local and national le vels. Roh is a Military Academy classmate of former President Chun Doo Hwan, and was one of the top leaders of the coup that brought Chun to power at the end of 1979. Yet since being elected president with just one-third of the popular vote, Roh has built up a reputation as a conciliator. In a country inured to noisy clashes between opposing personalities and points of view, this is no small achievement. Student protesters, and a fringe of intellectuals who back them, want the United States out of Korea. They also want unification on something close to North Korea's terms. Their demands are not entirely irrelevant, for as economic friction with the US rises, even the man in the street shows irritation at aspects of American policy. Gratitude for the American military presence since the Korean War is wearing thin. DESPITE all the problems South Koreans see Germany having with reunification, their thirst for a reunited Korean peninsula remains. The disagreement between student extremists and moderate citizens is not over reunification itself, but over the timing and the terms. Yet, a little over four years ago, the acrid smell of tear gas was almost a permanent fixture of the Seoul scene, and many ordinary citizens supported the students. To explain how South Korea got from there to here, one must delve further back into history. The individual who shaped South Korea more than any other postwar leader was Park Chung Hee, the bantamweight general who took power in a military coup in 1961 and remained president until assassinated by his own CIA chief 18 years later. Under Park, South Korea looked to its military defenses. But it also expanded its economy manyfold. Park did not waste money on grandiose steel mills (at least, not at first), but started with simple things - textiles, shoes, and cheaper electrical goods. He kept as close watch over his country's export performance as he would over a military campaign. Park was a nationalist, not a Jeffersonian democrat. Only under severe American pressure did he submit himself to the voters in multiparty elections. He went through the process three times before imposing an authoritarian constitution in 1972. Unlike the Marcoses of the Philippines, Park did not enrich himself at the country's expense, though he corrupted others with money and positions. His great achievement was to restore Korean national pride, damaged by years of harsh Japanese colonialism and postwar American tutelage. When Park was assassinated, his charisma had waned. Many Koreans were more than ready for the democracy that would complement their exuberant economic performance. When another coup occurred, this time led by Chun and his associates, the popular sense of betrayal was great. It was during the Chun years that student protests reached their climax. And it was Roh, now president, who calmed the protests by asking Chun to hold multiparty, one-man-one-vote elections for the presidency. So, overall, South Korea is a political and economic success story - something to remember the next time you see firebombs and teargas cluttering up your TV screen.