THERE CAN HAVE BEEN few occasions of more unbearable anguish and joy than the reunions last weekend of members of Korean families separated for 30 or more years as a result of the Korean War. The stunningly large figure of 10 million of the two Korean states' 60 million citizens belong to families divided by the Demilitarized Zone. It took decades of silent waiting and months of fitful negotiations for the Red Cross societies to agree on sending 50 of those 10 million -- one fortunate soul for every 200,000 Koreans -- across the border in each direction. At that, relatives of only two-thirds of these 100 could be located. One struggles to comprehend the disappointment of a Korean who came to meet a parent or child he had not seen or heard from since 1950, and found no one.

From the doleful experience of Korea and the other countries left divided by World War II -- China, Vietnam and Germany -- we know that the communist-ruled side of each of them isolates its people as a matter of political control and, when the time comes, as a matter of diplomatic bargaining. The communist side knows that people on the free or freer side not only ache to make contact with their kin but are in a position to influence their government to pay a certain price for it.

In the Korean instance, the North, running a regime sordid even by communist standards, created the separation of families by its invasion of the South in 1950, and subsequently perpetuated it in a manner permitting no mail, no telephone calls, no Red Cross missions, no hope. Only to keep partial pace with changes in its region has it relented slightly and symbolically over the last year.

The two caravans of Koreans and their escorts passed back across the DMZ at the end of the weekend without any plans having been made known for further North-South family contacts of any kind. This is awful, and most of the fault lies with the North. Each Korean government puts itself forward as a champion of humanitarianism and, beyond that, as a champion of eventual reunification. But the South is in relative terms an open society and the North is a closed one. The South is perhaps no less wary than the North of being exploited in a diplomatic exchange, but the South need not fear, as the North evidently does, the brief exposure of some of its citizens to life on the other side of the DMZ.