THE WORLD FROM...Tokyo

NTIL recently, Japan practiced diplomacy as if it did not have an enemy in the world. It saw itself as a behind-the-scenes mediator and a peace-loving nation seeking global disarmament.

Over the 48 years since the end of World War II, Japan has pursued peace and prosperity under the protection of the United States nuclear umbrella. As the only nation to suffer atomic strikes, it barely flinched as other big Asian powers - the Soviet Union, China, and India - developed the bomb over the decades.

But with unusual directness, Japan warned last month that North Korea had enough bomb-grade plutonium to build a nuclear weapon. It also noted that the Communist regime in Pyongyang could easily launch a bomb on a Scud-B missile across the narrow Sea of Japan.

Naming a potential adversary has not been a practice of Japan's post-war military doctrine. But ever since the collapse of the Soviet Union, North Korea has become the "foremost security threat," Japanese officials say privately.

At the same time, they see the US nuclear umbrella as providing less and less protection, both in terms of practical defense and political leverage, against such threats as a North Korean Scud.

Tokyo suddenly has the jitters, although it is hard to detect. During a visit last month by leaders of the US Congress, Prime Minister Kiichi Miyazawa startled them by wanting to talk mainly about North Korea, not about trade issues.

North Korea's announcement March 12 that it will pull out of the Nuclear Non-Proliferation Treaty (NPT), and thus end international inspections of its nuclear facilities, has sparked Japan into bold diplomatic action.

Japan quickly asked China to intervene and keep North Korea in the NPT, and began to tighten its screening of exports, including such items as sewing-machine tools, that could be used by the North Korean military.

"The simple fact is that the world cannot afford to let Pyongyang develop and possess nuclear weapons in this post-cold-war era, where arms reduction is such a pressing need," stated a March 14 editorial in Japan's Mainichi newspaper.

Some Japanese officials have expressed a conspiracy-like theory that South Korea might be too easy on North Korea in hopes of taking over the North's nuclear program someday when the Communist regime collapses.

A unified Korea with a nuclear weapon would be Japan's ultimate worry. Looking at a map, the Japanese have historically seen Korea as a knife aimed at their country's back. Of course, this analogy is the opposite of historical fact. Japan has invaded Korea at least twice and even tried to impose Japanese culture on Korea during a 35-year colonial period in this century.

But even more worrisome than a military threat is the pressure that a nuclear Korea might cause Japan to have its own nuclear weapons. While the idea remains anathema to most Japanese after Hiroshima, officials say they can no longer rule it out.

North Korea, for its part, claims Japan is making nuclear weapons from its advanced program to tap the power of plutonium for domestic energy.

The accusation, vehemently denied by Japanese officials and even by South Korea, was made after Japan refused to provide official recognition and much-needed economic aid to the North during negotiations that began in 1991 to normalize ties.

Suspicious of North Korea's intentions, Japan is using its economic clout to stop a potential enemy before it becomes one.