The only constraints on the existence, size, and weaponry of Japan's armed forces are self-imposed. Can other countries, especially the United States, rely on these constraints? Can the Japanese people trust them?
These are the questions at the heart of a seemingly esoteric controversy over whether Japan's defense spending should be allowed to go beyond 1 percent of gross national product on an annual basis.
Prime Minister Yasuhiro Nakasone has pledged that the defense budget being prepared for the fiscal year beginning April 1, 1985, will not exceed the 1 percent ceiling. But he has not denied that unless the national economy grows by an extra-ordinary percentage next year, the ceiling will inevitably be pierced.
2 Defense will be one important topic of the talks Mr. Nakasone plans to have with President Reagan in Los Angeles Jan. 2. Japan's defense budget stands at 2. 93 trillion yen ($11.86 billion), an increase of 6.6 percent (adjusting for inflation) over the 1983 defense budget. It is 0.991 percent of this year's GNP.
Both the Reagan administration and the US Congress consider this budget far too small, given Japan's commitment to defend its sea lanes out to a distance of 1,000 nautical miles. Arguments about Japan getting a ''free ride'' on defense surface periodically in Congress, which sees Japan enjoying a $30 billion trade surplus with the US.
The 1 percent ceiling was set by the dovish Cabinet of Takeo Miki in 1976, but it had been observed de facto for more than a decade before that.
While this ceiling enabled its Self-Defense Force to grow at a respectable rate, Japan's defense budget is still about half that of West Germany or France. This would be all very well if Japan were a small neutral country like Finland or Austria, the argument goes. But it is not. Japan is a major US ally on Moscow's Asian doorstep. The US-Japan security treaty obligates American forces to help Japan in case of attack.
Unlike West Germany, Japan is under no limits or constraints imposed on its defense capacity from the outside. West Germany, for instance, is forbidden by treaty to process or make its own nuclear weapons. Its defense establishment operates within the structure of NATO.
By contrast, the constraints on Japan's armed forces, including the so-called ''peace constitution'' renouncing war and banning ''war potential,'' are self-imposed. If Japan's voters decided tomorrow to amend the Constitution, to possess nuclear arms, or to build a defense force rivaling that of Moscow or Washington, no treaty would keep them from doing so.
It was all very well for Japan to shelter under the US nuclear umbrella when it was weak and rebuilding its economy. Now that Japan is strong, and conscious of its strength, it could decide, if it felt sufficiently threatened, to go nuclear, or to acquire a powerful offensive capacity, or to ally itself, say, with China.
Remembering history, many Japanese are uncomfortable about not having some agreed limit on the size of the military.
Perhaps the 1 percent ceiling is unscientific. Perhaps the so-called three nuclear ''nos'' - no to making, having, or bringing in nuclear weapons - are impractical in an age when the line between nuclear and nonnuclear battlefield weapons is becoming exceedingly thin.
Yet repeated soundings show Japanese voters comfortable with both of these constraints on their defense forces. They admit the need for defense. They want to keep it small.
Many suggestions have been made for replacing the 1 percent ceiling with a less constraining formula, such as ''about 1 percent'' or even ''1.5 percent.'' So far no one has come up with anything approaching the instant recognizability and quantifiability of the present ceiling.
By putting off the decision for one more year, Nakasone appears to be laying the groundwork for a new consensus over constraints on defense spending.