To what extent is Japan willing to use its economic power to help defend the security of East Asia?
The level of economic aid Tokyo decides to grant South Korea will test whether, as one South Korean defense expert put it, the Japanese are willing ''to put their money where their mouth is.''
By that, he meant that Tokyo should show to what extent it intends to back up its frequent assertions that South Korea is ''vital'' to the peace and security of East Asia by something more substantial than words.
If, because of its so-called ''peace constitution,'' Japan cannot make a significant military contribution to East Asia's peace and security, then let it make a substantial economic contribution instead.
The United States, committed by treaty to the defense both of South Korea and of Japan, frequently has urged Japan to step up its own defense efforts.
Secretary of State Alexander M. Haig Jr. has commended Prime Minister Zenko Suzuki for pushing through a 7.75 percent increase in Japan's defense budget this year despite restrictions limiting social security expenditures to a mere 2 .2 percent increase.
Nevertheless, Japan's overall defense expenditures come to less than 1 percent of its gross national product, compared with the 6 percent smaller and less prosperous South Korea spends on defense.
''We are Japan's defense shield,'' South Korean officials continually point out.
Furthermore, South Korea has a perennial trade deficit with Japan. Last year, the deficit reached $3 billion. Japanese investment in South Korea, after peaking at $220 million in 1978, has declined steeply. Prominent companies such as Honda, Matsushita Electric, and Mitsui Shipbuilding have pulled out of South Korea.
The reasons -- inflation, the wage spiral, and uncertainty about the government's industrial policies -- may be quite valid from a private investor's viewpoint.
But the overall picture emerging in Seoul is of Japan, a former colonial overlord now become an economic superpower, seeking its own selfish economic advantage but dependent on others for its defense.
Tokyo has indeed stepped up its defense efforts, but since Japan's armed forces are constitutionally limited strictly to self-defense, its main contribution to peace and security must be economic.
Thus, it has pledged to double official development assistance over a five-year period. Its total aid for the fiscal years 1982 through '86 should reach $21.4 billion.
Tokyo has also participated in aid programs for Turkey and Pakistan, two countries far from Japan but vital to the West in strategic and security terms.
How about aid closer to home, South Koreans ask.
What Seoul wants - and has been pressing Japan since summer to agree to - is a $10 billion, five-year aid package, of which $6 billion would be government loans and $4 billion export-import bank financing.
Last year, Sunao Sonoda, who was then foreign minister, brusquely rejected the Korean request, saying that the amount was far too large and that in any case Japanese budget procedures allowed aid to be given only on an annual, project-by-project basis.
This year, however, the climate seems to be improving. Prime Minister Suzuki told a New Year's press conference that he would ''make efforts'' to solve the economic cooperation problem between Tokyo and Seoul.
His new foreign minister, Yoshio Sakurauchi, has assigned high priority to ameliorating the strained atmosphere between the two capitals. Tokyo and Seoul are still far apart in terms of the size of the final aid package, but the Japanese seem to be moving toward accepting the need to make an overall commitment rather than doling out aid on an annual, project-by-project basis.
The Japanese are now talking about $1.5 billion in government loans and from Meanwhile President Chun Doo Hwan's appointment of a a new Cabinet headed by Yoo Chang Soon and including an impressive array of economic and financial talent may bring Tokyo and Seoul even closer together.
So the New Year has dawned on better prospects for Korean-Japanese economic cooperation. Tokyo expects shortly to send a high-ranking Foreign Ministry official to Seoul and then to work toward a meeting between the foreign ministers of the two countries, at which an aid figure acceptable to both sides can be reached.