London — When strategists spin their complex debates about the military balance of the United States and the Soviet Union, they usually discuss the global stalemate of nuclear weapons.
Yet this year, a rather more immediate trend emerges from one of the world's most respected groups of strategic thinkers.
The London-based International Institute of Strategic Studies sees the world experiencing a sharp buildup and growing refinement of ordinary, conventional, no-nuke bombs and bullets, tanks and planes, rifles and grenades.
That is not to say that the institute does not still argue and worry about shifts in the nuclear balance.
This year it is making news by arguing in its annual publication ''The Military Balance'' that the factual details of nuclear weapons available to each side contradicts the accepted Kissinger-Reagan thesis in the US that the Kremlin has a nuclear advantage in the first part of this decade.
No ''window of vulnerability'' at all, says IISS deputy director Col. Jonathan Alford. In fact, he goes on to insist, there never was one. The Soviets were never really able to launch a strike at US missiles in their underground silos without fear of an American counterattack.
Counting warheads only, American and Soviet missiles launched from ground and sea were close to equal, at about 7,000. Add in weapons delivered by bomber, and the US figure jumped to 9,300, while the Soviet total rises to only 7,300.
Switch to counting the amount of explosives carried by the warheads, and the Soviet land- and sea-launched missiles advantage was 2.65:1. Add in bombers, and the advantage drops to 1.6:1. Unless all the Soviet missiles were fired simultaneously, some US counter-missiles would get through.
Yet IISS officials dwell heavily this year on conventional weapons. While only two nuclear bombs have ever been dropped in a war (by the US on Hiroshima and Nagasaki), ordinary weapons are being shot off somewhere in the world every day.
Massive arms sales are being made to the third world. ''Arms don't necessarily cause wars,'' Colonel Alford remarks, ''but you can't have wars without arms, and there must be significance in the frequent transfer of arms into the Third World.''
As for the superpowers, the sharp buildup in Soviet conventional arms raises fundamental questions for those in the West who advocate unilateral disarmament, says the institute's new director, Dr. Robert O'Neill.
The Soviet stockpile of conventional arms also challenges those who advocate that NATO should never be the first to use nuclear weapons, Dr. O'Neill believes.
It is the superpowers that generate and purchase conventional arms in the biggest quantities. The institute detects a tendency by Washington and Moscow to buy the same kinds of conventional systems these days. Both are reinforcing their ability to wage war short of nuclear, and supplying their own client-states with arms.
Both are investing in new long-range aircraft (a new policy for the Soviets). Realizing that satellites don't always provide information quickly, both are buying strategic reconnaissance planes. Both want more planes built for a specific purpose rather than fighter-bombers combined. Three new Soviet planes have similar aspects to existing USaircraft. The ground attack SU-25 resembles the US A-10. The Mig-25 Foxhound has the same look-down, shoot-down ability as the US F-15. The Soviet blackjack bomber is similar to the US B-1.
The US, for its part, is following the USSR by introducing a genuine infantry combat vehicle. The US is also buying the same kind of many-barrelled rocket launcher that has been in the Soviet armed forces for four decades.
Both are boosting their navies.
The most positive aspect of the IISS report on the East-West conventional balance in Europe is that ''there would still appear to be insufficient overall strength on either side to guarantee victory.''
In the testing ground of the Middle East, Israel, using US planes and equipment, has routed Soviet-made air defenses and planes used by Syria in Lebanon. But don't jump to conclusions, Colonel Alford warns.
The Syrians have failed to operate Soviet air defenses properly. The Israelis have made brilliant use of American weaponry. But that is not to say that Russian soldiers would not operate their own equipment more efficiently. Israel has the edge over Arab opponents in training, especially of pilots. Put Soviet pilots in their own planes and that edge would shrink. Conventional war today often depends on the ''electronic balance'' - the ability of one side to link all its electronic systems together while blinding the enemy's, as Israel was able to do against the Syrians. Americans might have a harder time doing it against Russians.
In Europe, the institute says, the conventional numerical balance over the last 20 years has slowy but steadily moved in favor of the East. At the same time, the West has largely lost the technological edge that allowed NATO to believe that quality could substitute for numbers.
Outside the scope of the latest study are such vital factors as training, morale, leadership, tactical initiative, and geographical advantage. There is simply no accurate way of balancing one superpower's capacity against the other's.
The institute does make some specific comments. For instance, it warns that ''the Warsaw Pact's aircraft appear to be better able to survive and penetrate to their targets than NATO's. . . . (They) are generally newer. . . . And pact air defenses are somewhat denser.''
Dr. O'Neill warns that the Soviet SS-20 missile system, with a range of 5,000 kilometers, is being produced so rapidly that Warsaw Pact's ''arriving warheads'' of all kinds total 1,085 against NATO's 563.