ONE OF THE GREAT postwar turns in world politics was consummated in Tokyo on Monday when Japan and China put into effect a "treaty of peace and friendship." The significance of the event goes beyond the fact that two neighbors have forsaken hostility and hatred. The treaty also marks Japan's decision to stop trying to remain "equidistant" from China and Russia and instead to lean to Peking. Thus has the nation with both Asia's most powerful modern economy and its closest American tie made its most fateful choice since the war.

"Americans can view closer Japan-China relations without nervousness," George Packard and William Watts write in a new Potomac Associates study. "Each side is too committed to its own economic and political system and too nationalistic to join forces against the rest of the world; there is no need to worry about an East Asian 'Yellow Peril'." We would go even further. The Peking-Tokyo connection is a force for stability in east Asia and throughout the Pacific basin. Economically, the two nations complement each other. The Japanese figure they are in on the ground floor of a tremendous piece of business; the Chinese feel they have locked up the where-withal for the modernization they now devoutly embrace. Politically, the two should find common ground in seeking close ties with the United States and in keeping Soviet pressures at bay.

Why did the Kremlin beat out the Chinese in the contest for access to Japan's immense economic and strategic resources? The Soviets had decades in which to do the one thing - return four islands taken from Japan in World War II - that would have opened the way to normalization of relations. As a nation formed out of territory captured from neighbors, they apparently did not want to set a precedent of return. Some in the Kremlin expected that, when Peking finally began looking outward again for economic aid, it would look first to Moscow. That the Chinese chose Tokyo will surely raise in the kremlin the question "Who lost Japan?"

The United States must still tend carefully its separate relations with China and Japan. But their commitment to each other should facilitate that task. It is a development that adds to American security without giving the Soviet Union grounds for fair complaint. Americans can take comfort in the fact that, barely three years after the collapse of American power in one corner of Asia, the American position in the main arena is strong.