HAROLD BROWN has been in South Korea on what is superficially, for a defense secretary, a most unmilitary mission. He has been expressing the administration's concern for Kim Dae Jung, the civilian politican who was handed a death sentence for "sedition" by the regime of Chon Doo Hwan last September. In the months since, the United States has maintained a virtual boycott of high-level contacts with South Korea.It has sent Secretary Brown now to make the point that there is an intimate connection between the American stake in Korea's security and the performance of President Chon in this case. The connection lies here: the American commitment to South Korea flows not merely from a shared perception of international menace but from a shared devotion to certain secial values. Korea is no Jeffersonian democracy -- far from it -- but the kiling of a popular politician would be too much.

Some South Koreans evidently greeted the election of Ronald Reagan with considerable relief, thinking that he would set the Kim question aside for the sake of closer security ties. But the president-elect nicely upset those calculations by sending private word that the execution of Mr. Kim would have serious consequences for relations with the United States. This intervention could not have been more welcome and timely, precluding as it did the possibility that President Chon could make an end run around Jimmy Carter. Mr. Reagan has often been critical of the way Mr. Carter handled human rights. In at least this instance, however, he found an effective way to reinforce the Carter concern. The Koreans have received similar messages from Japan and other friendly countries. They know that the execution of Mr. Kim would do great harm to their international position.

For President Chon, it cannot be an easy thing to spare the life of the figure he as made a test case of his own stern resolute style. Like-minded supporters, especially perhaps among the officer corps whence he came, may wonder if the granting of clemency will not tempt others to think it is safe to challenge his rule. Nationalistic Koreans may not like the idean of acting in response to foreign pressure. Surely, however, the new South Korean leadership is mature and confident enough to see its interest in broadening rather than narrowing, the ground it shares with its leading allies and patrons.