''The Japanese-US defense relationship today is stronger than it has ever been,'' said Adm. Robert Long, commander in chief of United States forces in the Pacific.
''We are doing things that would not have been possible politically four years ago,'' the four-star admiral said in a recent interview. Among these are:
* A strong dialogue on defense planning.
* A joint study on the defense of sea lines of communications (SLOC).
* ''Increasing closeness of cooperation in the entire security area,'' including regular joint exercises among all three services of the two countries.
The SLOC study is particularly important, according to Pacific command sources, because it will flesh out what Japan must do in order to fulfill former Prime Minister Zenko Suzuki's 1981 pledge that his country would take over responsibility for defending its sea lines out to 1,000 miles. The study is expected to be completed by the end of this year. Admiral Long applauded the Japanese for increasing their defense budget each year despite economic recession, but added that he thought Japanese defense forces were ''inadequate.''
''I continue to strongly urge them to increase their defense budget - not for offensive capacity, but strictly for the defense of Japan. Also there is a need to increase the ability to conduct combat for a longer period of time,'' said the admiral.
Faced with severe budgetary constraints, Japan's self-defense forces have emphasized modernization of equipment at the cost of sustainability in combat. For instance, the air force is being equipped with the F-15 Eagle fighter while the navy is getting the PC-3 Orion anti-submarine patrol plane. But ammunition and missiles are in such short supply that many experts believe the Japanese forces lack credibility as a deterrent force. It is only the link with the US via the security treaty that makes Japan's defense viable, in this view.
Admiral Long said that the argument of modernization vs. sustainability is a continuing debate in defense establishments around the world, including in the US.
As the US field commander in the Pacific, he has worked hard to improve the sustainability of American combat forces. Admitting that on this matter there is often a difference of opinion between ''field commanders such as I and people at headquarters in Washington,'' he made clear his primary concern was ''being able to fight with what I have.''
Applying this perception to Japan, the admiral said it was fair to say that ''the Soviet threat is perceived essentially the same by both the US and Japan. The difference is in the urgency of the need to meet the threat. We believe the threat is not something in the long term. It is here now and in the near term.''
Japan currently spends about $11 billion a year on defense, or not quite 1 percent of its GNP. This is the eighth largest defense budget in the world, and in Asia is second only to that of China. But Japan's Constitution forbids war-making and its self-defense forces are minuscule compared to those of its giant neighbors, the Soviet Union and China.
The US Congress has frequently expressed dissatisfaction at the ''free ride'' Japan is accused of enjoying at the expense of the American taxpayer. Admiral Long did not use the term ''free ride,'' and indeed he said he was reluctant to give advice in public to the Japanese or to any other ally.
But the commander ''made it quite clear'' that he was concerned about the ''sustainability of Japanese forces.''