Japanese Prime Minister Yasuhiro Nakasone has taken a significant step toward defining his country as a member of the Western alliance. ''We don't want the Soviet Union to use Asia as a garbage dump for any SS-20s it may withdraw from Europe,'' Mr. Nakasone said at a press conference for Japanese journalists following the seven-nation Williamsburg summit May 30.
He indicated that this was one reason he had associated Japan with the six Western partners in a statement affirming determination to ''maintain sufficient military strength to deter any attack, to counter any threat, and to ensure the peace'' while calling upon the Soviet Union to ''contribute constructively'' to arms control negotiations. ''The security of our countries is indivisible and must be approached on a global basis,'' the statement said.
Britain, Canada, France, West Germany, Italy, Japan, and the United States are the seven nations which participated in the Williamsburg summit hosted by President Reagan May 28-30. The summit meant different things to each participant. For Mr. Nakasone and his country, it was a kind of international coming-out party on defense issues.
Japan has taken part in every summit of the West's leading industrialized democracies since France convened the first one at the Chateau de Rambouillet outside Paris in 1975. But Japan is the only participant which is not a member of the North Atlantic Treaty Organization. Furthermore, Japan's defense forces are restricted by its no-war constitution strictly to self-defense. Japan does not even accept the resort to collective self-defense permitted by the United Nations charter.
Thus every previous summit has been meticulously prepared in such a way as to concentrate on economic subjects. Williamsburg is the first summit to take up defense as an issue on which to make a joint statement, and it was able to do so because of President Reagan's insistence that he and his colleagues be free to discuss whatever they wanted without being locked into a tightly prearranged agenda.
For his part, Mr. Nakasone came to the summit determined to show a Japan that , in his words, was not ''just an economic animal,'' that it was ready to speak out, to take stands, and to shoulder responsibilities on matters important to the survival and prosperity of the global community.
The Williamsburg summit's main concern, as expressed in its final statement read out by President Reagan May 30, was to obtain a better coordination of Western efforts to promote world economic recovery by pursuing ''a balanced set of policies'' designed to obtain inflation-free growth, lower interest rates, reduced budget deficits, higher productive investment, and greater employment opportunities.
But arms control was very much on the minds of West European leaders. And the participants could not ignore the drumfire of Soviet threats, designed to divide the allies and stop the scheduled deployment of nuclear-tipped, intermediate-range American Pershing II and cruise missiles in Europe later this year.
Japan could not avoid being concerned, because the plan to deploy the missiles responds to the steady increase of mobile multiheaded Soviet SS-20 missiles aimed at Western Europe. These missiles have also been deployed in Asia , and Japan, like its neighbor China, does not want any East-West negotiation that might reduce SS-20s in Europe to lead to an increase of such weapons in Asia. The point of any talks between Moscow and the Western allies should be to eliminate these weapons altogether, not to move them from one theater to the other, both the Japanese and Chinese say.
In this sense Japan applauds President Reagan's zero-option proposal. Japan also accepts the NATO proposal for an interim solution that would reduce the number of SS-20s as well as the Pershings and cruise missiles planned to be deployed in Europe.
It is not yet clear what effect such a solution, if reached, would have on the balance of East-West forces in Asia. That is why Mr. Nakasone came to Williamsburg determined to get across the point, if the issue was raised, that security must be tackled on a global basis. As one Japanese official put it, even though Japan is not a member of NATO, the SS-20 has made security a global issue, has given it a dimension that transcends regional alliances.
This is a far cry from 1977, when West German Chancellor Helmut Schmidt voiced his concern over the SS-20 to his obviously uncomprehending Japanese counterpart. ''What is the SS-20 anyway?'' the prime minster was later reported to have asked one of his aides.
Mr. Nakasone said he made five points in discussing the matter with his colleagues at Williamsburg. First, the summit should express the common will of the participating nations. Second, to preserve world peace, deterrence and arms control negotiations must go hand in hand. Third, in order to induce the Soviet Union to talk, the Western nations must not change their plans to deploy Pershings and cruises at the end of this year. The timetable must be adhered to.
Fourth, in order to promote a summit between President Reagan and Soviet leader Yuri Andropov, the Western nations must give President Reagan a firm foundation of support from which to proceed. Fifth, the West must show patience and persistence in talks with the Soviet Union.
No Japanese leader at a summit meeting has spoken out as resolutely and explicitly as this. The deployment of nuclear weapons anywhere is always a politically controversial subject. It is especially so in the case of Japan, which has a firmly established national policy neither to make, possess, nor allow the stationing of nuclear weapons on its territory. Mr. Nakasone has come under strong attack from opposition parties at home over his statements.
Japanese defense policy has not changed, Mr. Nakasone told reporters. Japan has not joined NATO and is not bound by NATO decisions. Japan relies on its own self-defense forces and its security treaty with the United States. But if it is not to be isolated in the world, it must speak out and take stands on issues vital to its security.
''Western unity can have a political result - to bring the Soviet Union to the negotiating table,'' Mr. Nakasone said. ''If we are not united, the Soviet Union may just laugh at us and never sit down to negoitate. As a realistic statesman, that is how I think.''