An invisible no man's land runs the length of Japan. There are none of the barbed wire, mine fields, and array of weaponry apparent along the 38th-parallel demilitarized zone dividing North and South Korea. But the gulf between Koreans living in Japan who owe loyalty to either Seoul or Pyongyang is just as deep and seemingly unbridgeable.
Perpetuating the divisions cemented by the Korean war and continued state of high military alert, politics will largely determine where most of the 670,000 Koreans in Japan today live, go to school, and work, as well as whom they marry.
If there is one issue that can bring the two sides together it is the sense of being discriminated against by Japanese, a legacy of the deep hatred engendered by Japanese colonization of the Korean peninsula between 1910 and 1945.
These issues are highlighted by the visit to Tokyo this week of South Korean President Chun Doo Hwan.
Some 23,000 police will protect Mr. Chun each day from any North Korean-initiated assassination attempt. (One such bid in Seoul in 1974 against the late President Park Chung Hee, which resulted in the death of his wife, was carried out by a Korean resident of Japan.)
President Chun will likely raise the subject of better treatment for Koreans in Japan during his visit.
During its colonial rule, Japan imported hundreds of thousands of Koreans, some forcibly, to work in menial jobs in factories and coal mines. The Korean population rose from 40,000 in 1920 to almost 2 million in 1944. Many of today's Korean residents are descendants of these forced laborers.
Before the war, discrimination against the Koreans was common, including occasional pogroms. Second-class treatment continued after the war. One result of Japan's defeat was to strip Koreans of their Japanese citizenship. Those who could not or did not want to return to their Korean homeland found themselves stateless.
Emperor Hirohito formally expressed regrets to Chun Thursday for the sufferings inflicted on the Koreans during the occupation. The Emperor said, ''It is indeed regrettable that there was an unfortunate past between us for a period in this century and I believe it should not be repeated.''
For Koreans with allegiance to the south, the situation was eased by the establishment of diplomatic relations between Tokyo and Seoul in 1965. They were then able to obtain South Korean passports, and some have subsequently become Japanese citizens. But for the many tens of thousands still supporting North Korea, there are no such legal amenities: They are virtually stateless people.
To cater to the needs of the latter, the Chosen Soren (General Association of Korean Residents) was formed in 1955 with heavy financial support from Pyongyang (an estimated $155 million so far). It claims a membership of 210,000.
On the other side is Mindan, the (South) Korean Residents' Union, which claims to have 400,000 followers.
Whichever side they join, Koreans in Japan have had to struggle for a decent existence. Many live in the slum areas of Tokyo, Osaka, and other big cities. Largely denied decent job opportunities, they often have to eke out a living in the pachimko (pinball) parlors, bars, nightclubs, and cheap eateries in city nightlife districts.
This has spawned a major self-help effort by the Korean communities. Soren operates 154 schools throughout the country, along with a Korean university in Tokyo, which has a 92-member faculty and handles 1,250 students. In all, some 30 ,000 children are estimated to be attending Soren-sponsored schools today.
The North Korean community has its own credit unions and jobs provided by Soren-backed trading companies.
Similar opportunities are available for those loyal to Mindan. It has 36 credit-union branches throughout the country, for example, with total savings of some $3.5 billion at last report.
The two organizations keep their distance from each other. Violence is rare, although shots were fired at the Soren headquarters in Tokyo last year and several Soren high schools have been vandalized. It blames rightists funded by South Korean intelligence groups.
For years the Korean communities maintained a relatively low profile. But in recent times they have become more assertive, perhaps in line with the growing national pride at home.
Some Koreans have chosen to try to blend in with their surroundings, taking on Japanese nationality and Japanese names. Others are fighting againt the need for a name change.
Once it was compulsory. Now the Justice Ministry merely ''advises'' those applying for citizenship to take an ''appropriate'' Japanese name to better fit into society. To most applicants, that's as good as an order.
Within the Korean community a move is under way to fight this. In Nagoya, authorities are investigating a complaint by a Korean couple that they were pressured into giving their son a Japanese name before enrolling him in a local elementary school - a practice said to be prevalent in most of Japan.
Close attention is also being paid to the court battle of Sond Hae, who is fighting to recover the Japanese citizenship he lost after the war. Other Koreans are fighting a different court battle: against the requirement in the alien registration law stipulating that they be fingerprinted by local authorities. Koreans who were born in Japan should not be treated as ''aliens,'' they argue.
Assimilation is slowly taking place, however. A 1982 law allowing state-run universities to open their doors to non-Japanese scholars is finally paying off. Resident Koreans are being named this year as assistant professors at Osaka and Tsukuba universities.