'The threat'

''The Soviet threat'' has been asserted over so many months, by so many people, some of whom have obvious political or financial profit motives, and in such imprecise terms that just another phrasing of ''the threat'' tends to cause a yawn.

It is therefore with surprise bordering on excitement that I have been reading a new definition of ''it'' which is phrased precisely, carefully, and plausibly. It is the first thing I have read in a long time which persuades me that recent Soviet military developments have brought about a new condition calling for a new answer from the NATO allies, and that the allies ought to set about remedial action promptly.

This new phrasing is in the same book I was writing about in this space two days ago, ''Strengthening Conventional Deterrence in Europe,'' published this week by St. Martin's Press, New York, for the American Academy of Arts and Sciences.

One part of this book is a chapter called ''Soviet Operational Concepts in the 1980s.'' It is written by Christopher N. Donnelly, director of the Soviet Studies Research Center at the Royal Military Academy, Sandhurst, England. It identifies by ample documentation from Soviet military writers a detailed restructuring of Soviet armed forces in Europe aimed at the ability to paralyze NATO and establish Soviet control over Central Europe.

This would not necessarily involve actual war. The mere ability, if perceived by the European members of NATO, might achieve the purpose without actual fighting. The Soviet concept is tailored most carefully to avoid use of nuclear weapons. Mr. Donnelly says that ''even a limited exchange of nuclear weapons - if such a war is thinkable - with, for example, the U.K. or China, would present to the Politburo an unacceptable risk of social collapse.''

They want to avoid nuclear war, because their own internal structure could not stand the shock, but they also want to destroy NATO. How can they do that without risking nuclear war?

''The Soviet aim in a full-scale war against NATO must be to surround, destroy, or otherwise neutralize NATO forces, and thereby bring about a collapse of the NATO political structure within a matter of days - during the 'initial period.' An invasion can stop at nothing less than the occupation of West Germany, the Low Countries, and the Baltic Littoral.''

The technique would be ''a surprise attack on a broad front with several axes. The concept of the operation will be to insert on each axis, as an OMG (Operational Military Group), a strong division behind the NATO main belt of defence on the first or second day of the offensive.''

NATO would not be able to use tactical nuclear weapons. The Soviet forces ''inserted'' behind the NATO defense lines would be so close to and mixed up with NATO forces that it would be impossible to distinguish friend from foe. Besides, it would presumably take several days for NATO leaders to reach a decision whether to use nuclear weapons. By that time, if the Soviet strategic concept worked, it would be too late.

Could the Soviets achieve the surprise necessary to carry out such an operation?

Yes. In the Czech crisis of 1968 the Soviets achieved surprise by first causing a ''crisis,'' then ''defusing'' the crisis by a false agreement with Czech party leader Alexander Dubcek. When everyone was relaxed, the US President was in Camp David for the weekend, and much of NATO was on summer home leave, the Soviet Army rolled into Czechoslovakia.

Again, in the case of the Jaruzelski coup in Poland in 1981, timing and skillful management of the ''crisis'' produced ''almost total surprise.''

In both the Czech and Polish cases, Mr. Donnelly points out, the technique of crisis management was so well understood and practiced that the Soviets achieved almost bloodlessly their political purpose, which was to bring those two countries back under effective Soviet control. They use their armed forces, whether in overt war or without actual war, as a tool of policy.

Surprise is certainly possible for some further purpose, such as gaining control over West Germany. If they thought they could do it without risk of nuclear weapons being used - would they? They have always wanted to get control of all of Germany.

All of this is plausible. It is not ''Star Wars'' stuff. It could happen any day. NATO forces are not yet geared or armed to meet the kind of operation the Soviets are being trained to carry out. It would certainly seem that a restructuring and rearming of NATO is in order.