NATO's arms hold USSR in line. Some argue that the prospect of a scorched world has kept the superpowers' hands off their nuclear sword hilts. But how long can nuclear weapons maintain world peace?

Suppose you could wave your hand and rid the world of nuclear weapons now. Would you? Not so fast. Furthermore, suppose that the chance of nuclear war today is not very great, and that if all thermonuclear warheads were turned to dust, the possibility of a World War III fought with conventional weapons would greatly increase.

This scenario illustrates one of the main hurdles on the road to a nuclear-free globe. For decades it has been the official policy of the United States and its allies that if the Soviet Union invades Western Europe, NATO might retaliate with a nuclear attack against the Soviet homeland. This threat is perhaps the main reason there has been peace in Europe for 40 years, according to the conventional wisdom of nuclear theorists.

If both sides buried their nuclear arsenals, the Soviets would feel free to launch their massive tank armies on a march toward the English Channel. Given this scenario, almost every NATO government would keep its arsenal.

``I don't believe that we are near the point, or that we should be near the point, where nuclear weapons are discarded on all sides,'' said British Foreign Secretary Sir Geoffrey Howe last week. (West German Defense Minister W"orner on Reykjavik summit and arms, Page 6.)

Many people argue that certain aspects of the scenario aren't right - that the chance of nuclear war is greater than people assume, for instance, or that the Soviets aren't itching to send troops into West Germany. Kremlin leaders may not believe the West would actually use nuclear weapons first. This framework has shaped Western strategic thought for 20 years.

A main beam of this framework is the assumption that absent the nuclear threat, there would have been general war in Europe at some point since the end of World War II - the Berlin Wall crisis of August 1961 being a flash point often mentioned. In the modern era, Europe has gone longer without a major war only once, from the end of the Napoleonic Wars in 1815 to the beginning of the Crimean War in 1854.

``The awesome might of nuclear weapons has played a central role in keeping Europe a solitary island of peace, while the rest of the world was racked by some 150 violent conflicts and wars since 1945,'' concludes Carnegie Endowment analyst Josef Joffe in a recent article.

This point of view assumes that the conventional forces of NATO, its tanks, artillery, and fighter aircraft, would not by themselves deter a determined Soviet Union from war. Soviet conventional forces are larger, though the extent of their superiority is a matter of dispute. In the armed camp that is the European front, the Soviet bloc has a manpower advantage of 1.2 to 1. Soviet main battle tanks outnumber NATO's 2.6 to 1.

Building NATO armies up to parity would require enormous expenditures at a time when Western governments are cutting their budgets. Demographics further rule against it. Soon, West Germany, and to a lesser extent the US, will face a shortage of military-age youth. It is now virtually impossible for the West to match the Soviet Union tank for tank and plane for plane, according to former Defense Secretary James Schlesinger.

What NATO officials and analysts don't like to talk about is that even if they achieved conventional parity overnight, most would still favor keeping nuclear weapons. No number of new tank divisions can compensate for the fact that the Soviet bloc sits right next to Western Europe, and thus would have an easy time with supplies and reinforcements. The reserves of the US, NATO's bulwark, are thousands of miles away across the Atlantic Ocean. And in a sense, conventional weapons are just not terrible enough, say many analysts. It takes nuclear weapons and their prospect of a scorched world to keep the superpowers' hands off their sword hilts.

``Historically, conventional deterrence has been a rather unreliable mechanism for preventing war,'' concludes Johan Holst, director of the Norwegian Institute of International Affairs.

A large reason nuclear weapons have more deterrent value than mere guns is what is referred to as ``the crystal ball effect.'' In history, governments usually launched wars in a flush of confidence that victory would come on quick feet, and even if it didn't, the people whose lives were at risk were mainly soldiers.

Rational superpower leaders know that general war today means their nations laid into radioactive waste, with millions of their citizens dead, and thus even the most aggressive of them will stay their hand.

``I like the notion that East and West have exchanged hostages on a massive scale and that as long as they are unprotected, civilization depends on the avoidance of military aggression that could escalate to nuclear war,'' writes Harvard professor Thomas Schelling, one of the most influential theorists of the nuclear age.

In sum, most strategic theorists say scrapping nuclear arms now is impossible because it would make Europe safe for conventional war or - more subtly - for the Soviets to increase political pressure by making warlike noises.

The main criticism of this framework made by abolitionists, who would make banning the bomb an explicit governmental goal, is that it ascribes too much credit for peacekeeping to nuclear weapons. The reason there has been peace in Europe for the last 40 years, says antinuclear analyst William Arkin, is not that the Soviet Union is scared of US intercontinental ballistic missiles. It's because they simply have no stomach for trying to subjugate Western Europe. The Soviet Union has enough trouble with its Eastern European satellites, he says.

For the sake of a worst-case analysis that the Soviets would exploit any opportunity, the West has embraced, with enormous risks, a doctrine of reliance on nuclear weapons to protect Europe, say abolitionists. The gamble may be small, they say, but if the number comes up, it will mean Armageddon. Next: Would nuclear war mean the end of the world?