A make-or-break round of arms talks begins. The US and the Soviet Union are close to a historic agreement to cut nuclear weapons. Delegations meet today in Geneva to discuss removing medium- and possibly short-range missiles from Europe. The US's allies are on board for the medium-range proposal, but are wary of cuts in the short-range category.

THE most important arms control negotiations for the rest of this century open in Geneva today. They could, for the first time in the nuclear era, lead to an agreement to destroy thousands of nuclear weapons (instead of just putting a ceiling on existing numbers, as previous arms control agreements did). Or, if the talks fail, they could lead to an unprecedented all-out arms race in both defensive and offensive systems.

That this is the make-or-break round for the Reagan administration is obvious. Any arms control agreement that is to be concluded under this president has to be outlined in this session (or in a summit that would have to be held very soon); anything sketched out later would not be far enough advanced to survive the politicking of the 1988 presidential election.

That this is also the make-or-break session for the remainder of the 20th century is perhaps less obvious, but no less valid. There are two reasons for this. First, the still-popular President Reagan could carry a consensus for any agreement he was willing to sign. Any successor, however, whether Democrat or Republican, would face stiff opposition from the political right and probably could not muster a two-thirds ratification in the Senate.

Second, the only time when nations are ready to agree on mutual limitations on arms is when they enjoy rough parity, in a fairly stable environment, such as at present. Only then do they think that their security would be enhanced rather than degraded by arms control.

By contrast, in a situation in which one side leads or the balance might shift suddenly, the trailing side never wants to foreclose its chances of catching up, while the leading side never wants to give away its advantage.

Unless some agreed restraints are established now, in this period of relative strategic equality, just such an unstable race will characterize the 1990s.

The United States will forge ahead in the brand-new area of space defense and the Soviet Union will scramble to catch up - and the easiest way for both sides to outmaneuver the adversary's growing defenses will be to build more and more offensive weapons.

THE major question facing policymakers in the Geneva talks is therefore:

Assuming that Mr. Reagan wants the deep cuts in offensive strategic nuclear weapons that he says he does, is this goal important enough to him to accept the tradeoff of constraints on his pet Strategic Defense Initiative (``star wars'')?

The two are linked, since prospects are that strategic defense would be able to deflect a limited volley of warheads hurled against it, but not a larger one - and superpowers are therefore willing to limit their warheads by arms control arrangements only if they know the adversary is not going to mount a large strategic defense that would neutralize a limited arsenal.

For the US the obverse question must be: Is the US so confident that it can stay ahead of the Soviet Union in space defense over the long haul (and not just over the next ephemeral seven or eight years) that it would prefer a costly all-out arms race to mutual restraints? Or even if the military advantage might not be all that great, would it be worth it to force the sluggish Soviet economy into a competition that could bankrupt it?

For Soviet leader Mikhail Gorbachev, the looming question just now is: What will be the overall effect of his gamble in emphasizing Euromissile theater arms control at this stage over the more crucial strategic arms control? Will the agreement that is likely to be signed this year in this area of lesser importance begin momentum toward that larger goal that both superpowers endorse of 50 percent cuts in today's strategic offensive weapons? Or will the bonhomie surrounding the first superpower arms control agreement in eight years simply divert attention from the more critical question and remove the pressure to resolve it?

FOR both sides there is one additional question: even if both want a deal, could they accelerate the very tough nuts-and-bolts negotiations needed to produce an actual treaty fast enough to complete the bargain before Reagan's incumbency runs out?

In the central issue of strategic offensive arms control, both superpowers certainly have strong incentives to reach a comprehensive agreement - both to increase stability and safety in a dangerous nuclear world and to save the billions of dollars that would otherwise be spent in massive arms production.

Furthermore, Washington and Moscow are remarkably close in their broad positions and have been for some time; the Soviets have gone far toward accepting in principle the kind of inspection measures the US deems essential, and the one major unresolved issue (apart from any collateral restraints on space defense) is that of sublimits within the broad overall numbers of offensive warheads that would be retained under any 50 percent reduction in this category of weapons.

GLOSSARY FOR GENEVA Decoupling: strategic separation of American and European security via reduction of US commitment to defend Europe from Soviet aggression. Delinkage: Gorbachev's separation of INF from a comprehensive arms package (i.e., one dealing with strategic offensive and defensive weapons systems, including `star wars'). Deterrence: US commitment to possible first use of nuclear weapons, per NATO doctrine, to halt an impending victory of Soviet conventional forces in Europe. European theater: area in which any regional European war would be fought. Independent nuclear deterrent: the nuclear forces of France and Britain; these countries have their own weapons systems under their own control (i.e., not pledged to NATO defense). LRINF: long-range subgroup of intermediate-range nuclear forces; commonly referred to as `medium range' weapons, or as Euromissiles; range of 1,000 to 5,500 km (600 to 3,400 miles). Second zero option: removal of shorter-range INF from Europe; introduced by Gorbachev during US Secretary of State Shultz's recent visit to Moscow. SRINF: shorter-range subgroup of intermediate-range nuclear forces; range of 500 to 1,000 km (300 to 600 miles). Zero option: removal of all long-range INF from Europe; initially proposed by NATO in 1982; suddenly accepted by Soviet leader Gorbachev at the Iceland superpower summit in October.

LONG-RANGE INF The `medium range' missiles

Weapons in the long-range INF category (often referred to as medium-range) - where the Soviets maintain a nearly 3 to 1 advantage in Europe-targeted warheads - may be significantly reduced. The proposal being discussed calls for paring down these warheads to 100 for each superpower - and removing all medium-range missiles from Europe. If such an agreement is reached, the US and Soviet Union would each keep their remaining 100 warheads in areas out of range of the European theater. The US says it would be willing to eliminate all weapons in this category.

SHORTER-RANGE INF The `short range' missiles

If the medium-range missiles were removed from European soil, a significant imbalance would remain in the next category `down' of nuclear weapons. The Soviets maintain something like a 9 to 1 advantage in shorter-range INF systems if those deployed in the Soviet Union are included. The US has no weapons in this category, but it does control the warheads on the West German Pershing 1-As. The US has said it might meet any requirement to eliminate its long-range INF by removing a stage from Pershing 2 missiles, thereby making them shorter-range Pershing 1-Bs. The Soviets have not been receptive to this idea; they have recently offered to discuss the removal of all shorter-range INF from Europe.

CONVENTIONAL FORCES

Here lies the crux of the problem. In addition to worries about imbalances in nuclear weapons systems, the Europeans are nervous about the Soviet 2-to-1 advantage in various land-based heavy weapons. Many feel that nuclear weapons have kept the peace there for 40 years. The Soviets, say the allies, would never dare launch a conventional attack if NATO has nuclear warheads that could reach Soviet territory.

The peeling away of protective layers of long- and shorter-range INF, some say, would leave Europeans feeling vulnerable. The thousands of battlefield (or `tactical') nuclear weapons remaining would not have the full deterrent effect of weapons whose ranges reach into Soviet territory. A further dilemma for the allies is the huge expense that would be incurred if they had to beef up their conventional forces to match Soviet and Warsaw Pact numbers.

But any INF agreements would not denuclearize Europe. Intercontinental strategic forces would be left in place, as would nuclear-capable submarine- and air-launched missiles. Britain and France still would have their own, independent nuclear forces.

The superpowers seem ready for agreement. The NATO allies, while cautious, have approved the long-range INF zero option and in the shorter-range are not likely to let themselves be portrayed as standing in the way of a small but significant reduction in global nuclear stockpiles.