The United States wants Japan to play a greater role in defending strategic sea lanes in the northwest Pacific.
In his first tour of Japan and East Asia, US Defense Secretary Caspar Weinberger emphasized the importance of this task during his visit to Tokyo this past weekend.
Mr. Weinberger's approach was gentle. But in conversations with Prime Minister Zenko Suzuki and Defense Minister Soichiro Ito and in meetings with the press, he brought up Mr. Suzuki's commitment last year to defend Japan's sea lanes out to a distance of 1,000 miles.
Japan currently spends just under 1 percent of its gross national product on defense. If it were to increase spending so that it could defend these routes, US naval forces now committed to Japan's defense could spend more time in the Indian Ocean countering a mounting Soviet threat there.
That seems to be the reasoning behind the increased US pressure on Japan. The Weinberger approach rejects the attempts of some American legislators to link defense with trade issues -- the argument that Japan should increase its defense budget or buy more arms from the US as a means of reducing its trade surplus.
Mr. Weinberger does not talk in abstract terms about how great a percentage of its gross national product Japan should spend on defense. (The US spends 5.9 percent of its GNP on defense, while the policy of the Japanese government is not to allow defense spending to exceed 1 percent.)
Mr. Weinberger starts from the premise that Japan has already agreed, within the limits of its no-war Constitution, to ''defend its own territory, the seas and skies around Japan, and its sea lanes out to a distance of 1,000 miles.''
He then asks what Japan requires in order to fulfill this commitment. Looking at Japan's current defense budget and plans for the next few years, he suggests that, although the 7.75 percent increase in the defense expenditures this year ''is an encouraging first step in the right direction,'' Japan's forces today ''have not yet reached the point of being able to carry out their constitutional missions fully. . . .''
Mr. Weinberger did not publicly quantify the kind of procurement he thinks the Japanese self-defense forces will need. But the Japanese are uncomfortably aware that even the maximum increase in its equipment and personnel during the five-year buildup plan will probably be found inadequate by the Americans.
Japan's defense budget for the current fiscal year is 2.6 trillion yen or about $11.75 billion at an exchange rate of 220 yen to the dollar. (The yen has declined rapidly in recent weeks, to nearly 250 to the dollar, but many observers consider this a temporary phenomenon.)
The Defense Agency's three service chiefs -- representing the ground force, air force, and naval force -- are reliably reported to be requesting that the five-year buildup program (1983-87) totals some 5.6 to 5.7 trillion yen or about in the 1983-87 period could reach 20 trillion yen, or an average of $18.2 billion a year.
Mr. Suzuki is unlikely to approve these figures because they would take Japan's defense budget beyond the 1 percent of GNP that he has pledged to maintain. Last year, Japan's GNP was about $1.3 trillion. With the economy stagnating and tax revenues declining, Japan's GNP is not likely to record the spectacular increases it has in recent years. The government's finances are increasingly constrained.
Yet the commitment to defend Japan's sea lanes has been publicly stated.
''No one in the United States wants to see Japan become a military superpower ,'' Mr. Weinberger said in a speech to the Japanese National Press Club March 26 . ''But Japanese forces capable of providing sea and air defense in the northwest Pacific could complement US strategic and conventional forces in the area. . . . The vital arteries of free commerce in the Pacific would be strongly defended. Thus Japan can, within the constraints of self-defense, contribute to her own and, indeed, to global security.''
Will the Weinberger approach work? Popular opinion in Japan has come a long way from the utopian pacifism of the immediate postwar period. The Afghan and Polish crises, as well as Moscow's military buildup on islands within a stone's throw of northern Japan, have heightened public awareness of the Soviet threat.
But there is still considerable resistance to the idea of a defense partnership with the US in a narrow military sense, as distinct from a more general security or economic partnership. Consensus on defense issues is still a fragile blossom, and Mr. Weinberger's approach is expected to bear fruit only if he nurtures this blossom with gentle care.