Control of Soya Strait, a 24-mile stretch of water between the Soviet island of Sakhalin and Japan's Hokkaido, is the ''No. 1 priority'' for Soviet naval planners, in the view of Vice-Adm. M. Staser Holcomb.
One of Admiral Holcomb's jobs as commander of the United States Navy's Seventh Fleet is to make sure the Soviets never gain exclusive control over this strait nor turn the adjacent Sea of Okhotsk into a Soviet lake.
To this end, US Seventh Fleet ships periodically enter the Okhotsk Sea through one of the channels between the Kuriles, pass through the Soya Strait, and proceed on down the Sea of Japan.
The Soviets are not pleased. They would like to turn the Sea of Okhotsk into a secure hiding ground for their latest ballistic missile submarines, which are capable of reaching almost any target in the United States.
They must also worry about the Soya Strait as the bottleneck in the supply line that runs from Vladivostok on the Sea of Japan, through Soya and the Sea of Okhotsk, out to the Pacific port of Petropavlovsk.
The two principal bases of the 800-ship Soviet Pacific fleet are Vladivostok and Petropavlovsk. About three-fourths of this fleet is home-ported in Vladivostok, while one-fourth is in Petropavlovsk.
The advantage of Petropavlovsk is that it is an ice-free port on the open ocean. The most modern ballistic missile submarines of the Soviet fleet, the best attack submarines, and other ships capable of fighting the US Seventh Fleet are in Petropavlovsk.
But in wartime Petropavlovsk would be isolated. It can be supplied only by sea. Once the umbilical cord linking it to Vladivostok were severed, Petropavlovsk would eventually wither and die.
Thus, in Admiral Holcomb's view, Soviet naval planners must have detailed contingency plans for ensuring control of the Soya Strait in wartime.
''If in spite of all his (the Soviet naval planner's) preparations,'' Admiral Holcomb said in a recent interview, ''I could - in the dead of night - get the strait mined so as to destroy the first ship that tried to get through, he'd be in a terrible situation.''
The Japan Sea has three usable exits - Tsushima Strait between Korea and Japan, Tsugaru Strait between Hokkaido and Japan's main island of Honshu, and Soya Strait (also known as La Perouse after the European explorer). Part of the purpose of the current five-year Japanese defense buildup is to acquire the capacity to deny the Soviets the use of these three strategic straits.
Admiral Holcomb concedes that it would be next to impossible for Japan to gain 99 percent control over these straits. But even 50 percent control - the ability to sink at least every other ship trying to get through - would cause the Soviets terrible problems, he says.
If the Soviets were convinced they had no hope of getting through either Tsushima or Tsugaru Straits, Admiral Holcomb said, they might then focus all their efforts on Soya Strait, and even accept a high risk in order to ''burst through.''
The fatal deficiency, from the Soviet viewpoint, is that they control only the northern side of Soya Strait. The southern side is part of Japanese Hokkaido. There is a high degree of likelihood, therefore, that Soviet contingency plans provide for trying to take over at least the northern tip of Hokkaido. Japan's Self-Defense Forces are well aware of this possibility, and that is why Japan's best airplanes, its best ground equipment, and its most seasoned troops are all in Hokkaido.
If northern Hokkaido remains outside Soviet control, the next best thing, from the Soviet viewpoint, would be to heavily fortify the four islands of the southern Kuriles, the nearest of them only a few miles off the eastern tip of Hokkaido. These four islands are claimed by Japan as its ''northern territories'' illegally seized by Soviet troops after Japan's surrender in World War II.
At one time it looked as though the Soviets might at least return the smallest two of the four islands - Shikotan and Habomai (actually the Habomais are several small islets, not one single island). But no longer.
The two larger islands in Japan's ''northern territories'' are Kunashiri and Etorofu. The Soviets have a substantial military presence on Etorofu and now probably consider all these islands essential to their plans for wartime control of Soya Strait. Under these circumstances, Japan's chances of ever getting these islands back seem extremely remote.
The Soviets are also fortifying Shimushir Island halfway up the Kurile chain. In Admiral Holcomb's view this is part of Moscow's plan to make the Kuriles, as well as the Soya Strait, a barrier impenetrable by hostile forces.