For the United States and the Soviet Union, the northern Pacific is an increasingly important and sensitive strategic area where intelligence probing and the testing of military resolve are common.
Tragic miscalculation and callous overreaction may have played a part in the shooting down of the South Korean airliner last week, military and intelligence sources say. But growing US and Soviet arsenals, more military activity on both sides, and fear of attack in or from the region are the key underlying factors. US officials also cite a Soviet penchant for secrecy and ''paranoia'' over territorial intrusion as contributing to an atmosphere in which the recent confrontation occured.
In recent years, Soviet sea forces have grown substantially, marking a shift from what had been essentially a coastal defense force to a ''blue water navy'' that now includes aircraft carriers, guided missile cruisers, and several new classes of submarines. This applies to all the world's seas, but especially the Pacific.
''The Soviet Pacific fleet is now the largest of the four Soviet fleets,'' says Robert J. Hanks, a retired rear admiral and former director of strategic plans and policy for the US Navy.
Pointing to the Soviet takeover of former US facilities in Vietnam and the new Soviet carrier and battle cruiser groups now sailing the Pacific, he adds, ''They have a growing capability to put the ships out to sea and keep them there.''
According to US military and intelligence sources, the Soviet Union now counts 80 principal surface combatant ships, more than 30 ballistic missile submarines (40 percent of all Soviet strategic subs), at least 90 attack subs, more than 30 long-range Backfire naval bombers, and an 8,000-man naval infantry division (the largest marine unit in the Soviet military) as part of its Pacific fleet.
''The steady, year-after-year expansion of Soviet Navy capabilities shows no signs of slowing down,'' Rear Adm. John L. Butts, director of US naval intelligence, told Congress last March. ''The Soviet Navy now has enough modern ships and aircraft to provide a naval presence around the world and they are doing so on a day-to-day basis.''
Likewise, since the advent of the Reagan administration, US maritime forces and tactics have become more bristling as the US Navy hastens to achieve its goal of 600 ships, including battleships with cruise missiles and more aircraft carriers. In the words of US Navy Secretary John Lehman, this country is now quite willing to project its seapower ''into harm's way.'' This has become evident in the Pacific.
''We've been more aggressive with the use of the Seventh Fleet,'' says Stansfield Turner, former director of the Central Intelligence Agency and a retired admiral. ''It's part of an overall policy of showing the flag more, of being more visible, more willing to go into areas that are sometimes disputed.''
The same apparently is true of the Soviet Union. Last year, Soviet Backfire bombers for the first time flew simulated strikes against US aircraft carriers in the western Pacific.
''On two occasions, Backfires came within 100 miles of US carriers as part of the overall reaction to one of our exercises,'' Admiral Butts reported. ''While the Soviets routinely monitor such exercises and simulate various reactions, this was the first use of Backfires against our carriers. It was also the first use of Backfire over the open ocean in the Pacific.''
Growing naval might in the region has been accompanied by increases in other forces as well.
Soviet Army and Air Force facilities have been expanding on Sakhalin Island, just north of Japan. The US plans to deploy a wing of F-16 fighters, which are capable of carrying nuclear weapons, in northern Japan late next year.
In anticipation of this, the Soviet Union last month stationed its modern MIG-23 jets on Etorofu Island, close to the Japanese island of Hokkaido.
American maritime interests in the Pacific are aimed largely at preserving political stability in the region and preventing Soviet expansion.
Military analysts say Soviet concerns are twofold: protecting its eastern flank against possible Chinese attack in time of superpower conflict, and preventing its forces there from being trapped by geography.
Petropavlovsk, the Soviet Union's ice-free Pacific port on the Kamchatka Peninsula, is homeport for about one-fourth of the Soviet Pacific Fleet. It is linked by sea to fleet headquarters at Vladivostok on the Sea of Japan. Between the two are narrow straits or choke points that could be mined or otherwise attacked in wartime, blockading or cutting off supplies for most of the Soviet Pacific Fleet. Moscow naturally wants to prevent this possibility, while the US sees this vulnerability as a military asset to be exploited.
With the Soviet regional military expansion has come increased US intelligence activity as well as more fleet exercises. US reconaissance planes regularly fly in this area to keep an eye on Soviet military facilities and monitor Soviet air defenses. They also track the unarmed Russian strategic missiles that end their test flights on the Kamchatka Peninsula. A US Air Force RC-135 was on an intelligence-gathering mission about the time the South Korean Air Lines Boeing 747 was downed.
The airliner apparently flew near the Soviet port at Petropavlovsk, within the Sea of Okhotsk (where nuclear missile-armed Soviet subs lurk), and across the tip of Sakhalin Island on a course that headed for Vladivostok, all areas of critical military importance to Moscow.
The US on occasion has accused the Soviet Union of using commercial flights for intelligence-gathering purposes.
Moscow has charged the US with the same thing, and at first suggested the South Korean airliner was spying. Now, Soviet officials suggest the South Korean aircraft was mistakenly assumed to be the US intelligence plane.