FYI: What you need to know about The Bosporus Straits. Even in times of peace, the naval forces of major world powers are constantly engaged in a shadowy battle of maneuvers, designed to display and test their effectiveness. Turkey's Bosporus Straits play a key role in such moves -- both for the Soviet Union and Western nations belonging to the North Atlantic Treaty Organization.

The Bosporus Straits, which separate the European side of Istanbul from the Asian side, lie at the eastern end of a 25 mile-long seaway. This waterway connects the Black Sea to the Mediterranean via the Sea of Marmara and the Aegean Sea. The Dardanelles Straits mark the western end of the seaway, which is commonly referred to as the ``Straits zone.'' Istanbul has been a crossroads of trade and civilization for more than 15 centuries, and the Bosporus Straits have seen a constant flow of shipping. These straits provide the only access to the open sea for Bulgaria, Romania, and those parts of the Soviet Union bordering on the Black Sea.

The strategic nature of the Bosporus Straits was highlighted by two events earlier this month:

The Soviet Union's launching of its first full-scale aircraft carrier from a Black Sea shipyard.

The passage through the straits of two Soviet warships, amid rising tensions between the United States and Libya, a Soviet ally. The vessels were headed to the Mediterranean, where the Soviet Union normally maintains a fleet of 45 vessels -- including cruisers, destroyers, submarines, frigates, and supply ships.

For Turks living in the area, and accustomed to seeing Soviet warships sail by, such moves are of little interest. But for the Turkish military authorities, any increase of Soviet naval traffic -- as in the past month -- is of particular significance. The Black Sea end of the Bosporus is a restricted military area, and is heavily guarded by the Turks, who record all these movements and share the information with the US and NATO allies.

Although the Turks do not publicize these figures, it is known that Soviet naval and merchant traffic is growing rapidly each year. Many fishermen, who ply small boats in the Bosporus, report increased movements.

The growth of shipping through the Bosporus has created several problems: It disturbs fishermen and the ferries which ply between the European and Asian coast, carrying thousands of commuters, and causes substantial water pollution. But there is nothing the Turks can do about this sea traffic along the straits. Free passage is guaranteed to all ships under the 1936 Montreux Convention (see accompanying box).

At the time of signing, the Soviet Union was not a strong naval power. But since the end of World War II, strengthened Soviet forces have extended their presence and influence in the Mediterranean. Since 1964, both the quality and quantity of the Soviet naval presence in the Mediterranean has improved, an International Defense Review report says.

Immediately after World War II, Moscow requested a revision of the agreement, so that it would have control of the straits. Turkey rejected the Soviet demand with the support of Western signatories and the US. Soviet threats and pressures on Turkey at that time led to active Western support of Turkey and eventually to its admission to NATO in 1952.

Since then, neither the Soviets nor any other country have asked for a revision. It is clear that no one wants to disturb the delicate balance prevailing in this strategic part of the world.

The big question now, analysts say, is whether a full-scale carrier, like the one the Soviets launched earlier this month, can pass through the straits.

US and Western experts here argue that passage of aircraft carriers is barred under strict interpretation of annexes to the Montreux Convention. But, in 1976, the Turkish government allowed the Soviets to sail a Kiev-class warship through, much to the disappointment of the Western allies. The Soviets classified the ship, which carried helicopters, as an anti-submarine cruiser. But now, say NATO sources, such ships carry vertical or short takeoff and landing aircraft.

The Turkish argument then was:

Legally, since the vessel was formally classified as a cruiser, it could have free passage through the straits

Politically, Turkey did not want to create a conflict that might lead eventually to Soviet demands for a revision of the convention.

The question is whether these arguments will now be applied to the passage of a full-scale aircraft carrier.

The Soviets say yes. According to a Soviet Embassy spokesman in Ankara, under the second annex to the Montreux Convention, ``aircraft-carriers can also pass through the straits.''

The spokesman said that the Soviets act in compliance with ``the letter and spirit'' of the accord, adding, ``We reckon that Turkey does not want this convention to be violated in any way.''

Turkey has not commented on, nor hinted at, what its decision will be when the Soviets try to send the aircraft-carrier through the straits, as expected, in a year or two. But there is a good deal of argument and discussion on the subject among experts here.

Opinion seems to be divided. Some say the second and fourth annexes give the Soviets the right to send their carrier through. Others maintain that there is no clear or precise provision in the convention that authorizes this. Legal experts say there are loopholes in the convention that the Soviets could try to use in their favor.

Politically, the likelihood is that Turkey might bend toward letting the Soviet vessel go through the straits. A senior former naval officer said, ``The Soviets have not built this huge ship to keep it in the Black Sea. They are determined to send it to the Mediterranean and to other high seas.''

Could Turkey, which is at the moment trying to get economic advantages from its rapprochement with the Soviet Union, prevent this? Western observers here are doubtful.

When Turkey was criticized for allowing the Kiev-class vessel to pass through the Bosporus, a senior Turkish diplomat reportedly told NATO officials: ``If you want so much to stop it, go ahead and do it [yourselves].''

The Turkish reaction this time may not be very different. Rules of the Straits The Montreux Convention of 1936

This agreement returned the Straits zone, including the Bosporus and Dardanelles straits, which had been under international control to Turkey.

Under the terms of the agreement, Turkey is authorized to close the straits to warships of all countries when it is at war or threatened by aggression.

Merchant ships are to be allowed free passage during peacetime and, except for countries at war with Turkey, during wartime, too. The Black Sea powers, principally the Soviet Union, are authorized tosend their naval fleets throught the Straits into the Mediterranean in peacetime.

Black Sea countries must notify the Turkish government of the use of the Bosporus by any of their warships at least eight days in advance. There is no such obligation for merchant ships.

The convention was ratified by Turkey, Britain, France, the Soviet Union, Bulgaria, Greece, Germany, Yugoslavia, and -- with reservations -- by Japan. It has remained in effect despite Soviet efforts to obtain its revision.