Mons, Belgium — Just before dawn the Soviet 2 Guards Tank Army, with supporting Polish and East German elements, crosses the Elbe River south of Hamburg and mops up American covering forces and I Netherlands Corps.
By the end of ''D-Day'' the Soviet forces have seized Bremen and Kiel and sealed off Hamburg.
By D-Day plus three only half of 2d Lt. Andrei Nekrassov's 10 Soviet armored personnel carriers survive, and the rations that arrive include only 10 jars of meat paste for his entire company.
On the other side of the lines, RAF mechanic Brian Illingworth writes home to Yorkshire: ''There was an enormous bang on the roof . . . and a shower of paint and muck came down'' as Soviet planes dropped their bombs nearby. ''Corp and me and the other lads rushed out to see what had happened. We were a bit daft really, and Sarg didn't half tell us so too, because of course Ivan could have come round for a second go.''
* This is ''The Third World War,'' as imagined by Gen. Sir John Hackett. The novel is meticulously accurate in most details. Yet some critics think the former commander in chief of the British Army of the Rhine plays down NATO conventional (nonnuclear) inferiority and doesn't force on his fictional NATO commander and the American president the agonizing resort to nuclear weapons to avoid a conventional rout.
Is Sir John right?
Or is the present NATO commander, Gen. Bernard W. Rogers, right in preaching that if NATO doesn't beef up its conventional forces, the West would have to go nuclear - fast?
What both do agree on is that the risk of a third world war has grown to some extent with the Soviet military buildup of the past 20 years. Soviet-bloc tanks outnumber NATO tanks by almost 3 to 1 along the central front of the East-West German border area (plus Belgium, the Netherlands, Luxembourg, Czechoslovakia, and Poland). Soviet-bloc artillery, antitank guns, and air-defense guns also outnumber NATO equivalents by almost 3 to 1, multiple-rocket launchers by 7 to 1 .
Soviet superiority in conventional weapons in Europe is undisputed. It is a far more serious military threat than the more publicized strategic nuclear balance.
In the strategic equation, the Soviet Union's 8,000 intercontinental nuclear warheads threaten the United States. But America's 9,000 intercontinental warheads threaten the USSR just as much.
In the European equation, however, the Soviet bloc's 25,000 main battle tanks on the central front threaten Western Europe - while NATO's 9,900 tanks cannot conceivably threaten the Soviet bloc.
So what? an American might ask. Does it matter to the US if Europe stands under threat? Is Europe in fact worth defending? And if so, is it defendable?
For virtually all of the three dozen American and European military officers and civilian strategists interviewed for this series the answers to these questions were: Yes, yes, and yes.
Europe's importance to the US begins as a matter of roots. Most Americans stem from European stock. America's culture and its Enlightenment democracy are overwhelmingly European. Our shared values of freedom and individualism are worth protecting, generations of Americans have thought ever since 1917.
There are more hard-nosed reasons as well for American self-interest in the fate of Europe.
Europe is the richest geographic area in the world, with a combined gross domestic product of $3.1 trillion (NATO members only), as against North America's $2.9 trillion, Japan's $1 trillion, and the Soviet Union's close to $1 trillion. To both superpowers Western Europe was the greatest prize after World War II. Europe's domination by the Soviet Union would give Moscow such towering economic and military strength that there would no longer be two superpowers, but one alone: Soviet Eurasia.
As an American colonel at NATO military headquarters at Mons, Belgium, puts it: ''In trade figures, economics, Europe is our principal trading partner. In pure military terms, take the population, the pure economic capability of the Western alliance vis-a-vis the Warsaw Pact; the alliance has far more people, far greater economic resources, far greater military potential. ''But now if you take Europe away from that equation and add it to the Soviet balance, look at what that does to the population, resources, and military potential,'' he goes on. It becomes ''very stark'' how important Europe is. ''It's the critical thing. If it's on our side, we have the balance. If it's on their (the Soviets') side, they have the balance.''
Twice in this century the US accepted the colonel's reasoning and decided (belatedly) that it had a vital interest in ensuring that Europe not be dominated by a single expansionist, nondemocratic state. In both 1917 and 1941 the pace of war, as terrible as it was, was slow enough for the US to change its mind, get its industrial machine going, and turn the tide of battle.
Next time there will be no such grace period. Strategic planners agree that in an age of enormously destructive weapons the US is either committed to Western Europe in advance - the choice it made in the late '40s and has held to ever since - or else it writes Western Europe off to the one continental power that would like to dominate Europe: the Soviet Union.
This said, the aim of the US - and of NATO ever since its founding three decades ago - has always been ''deterrence'' rather than ''defense.'' That is, the very fact of the alliance and of America's prior commitment to defend Europe is intended to avert any Soviet attack by showing that an invasion would cost far more than it would win.
This approach has worked well, in the view of NATO member governments. It has by now secured peace in Europe for 38 years - a longer period than ever before in this century in this cockpit of both world wars. While armed conflicts have been an all-too-common occurrence around the globe in the past three dozen years , they have conspicuously not taken place in Europe - despite the unprecedented East-West confrontation here.
(The antinuclear activists argue that Moscow does not have any designs on Europe anyway, that its stance is essentially defensive. NATO governments say this is naive; that the Soviet takeover of Eastern Europe after World War II, the Berlin crises, and the Soviet invasions of Hungary in 1956, Czechoslovakia in 1968, and Afghanistan in 1979, and its more-than-defensive military buildup suggest otherwise.)
There is a catch, however, in NATO members' confidence in the deterrence they think has worked so well till now. This deterrence has always depended on a European military balance. But in the past decade the European balance has been tilting toward the Soviets.
The point here, officials say, is not that the Soviet Union is contemplating an attack on Western Europe. The point is that if (to use Soviet terminology) the ''correlation of forces'' were to shift so decisively in Soviet favor as to make an attack look like succeeding, then Moscow might find the temptation to exert political pressure on West Europe irresistible. This much was evident in Soviet leader Nikita Khrushchev's squeeze on West Berlin in the late 1950s - in a brief period when Moscow thought it had nuclear superiority - and in Khrushchev's concurrent threats to Italy, Greece, and ''the very existence of the population of West Germany.''
Hence the real Western concern about today's shift in the European military equation.
Basically, in the past NATO has always effected a military balance by relying on some form of Western nuclear superiority to offset its perennial conventional inferiority vis-a-vis the Soviet bloc. It has always threatened a last-resort nuclearm retaliation for any conventionalm attack.
Thus, in the 1950s and early '60s American policy endorsed ''massive retaliation'' on the Soviet Union for any otherwise unstoppable Soviet attack on Western Europe. And ever since the late 1960s - under the doctrine of ''flexible response'' - NATO has warned it would respond to any otherwise unstoppable attack with theater nuclear weapons (artillery shells, short-range missiles, or air strikes).
American nuclear policy in Europe is thus fundamentally different from American nuclear policy at the strategic (intercontinental) level. At the strategic level the use of nuclear weapons has been basically restricted to second use - to retribution for any first use of strategic nuclear weapons by the Soviet Union.
In Europe, however, the US and NATO plan on first use of nuclear weapons - and moreover, serious ''war fighting'' with these weapons - if deterrence fails. This is a policy fraught with moral anguish for any American president who would be the first to loose an exchange of the most terrible weapons of mass destruction known to mankind.
This anguish has only become more acute as the old American strategic nuclear superiority has waned even while the whole European equation also has changed. The West no longer possesses either the strategic or the theater nuclear superiority it depended on in the past to ward off any conventional attack in Europe.
The Soviet Union achieved strategic parity in the 1970s. And with its modernization of intermediate-range missiles beginning in the late '70s, it even achieved theater nuclear superiority in numbers of warheads. This modernization is symbolized by the mobile SS-20, with its 5,000-kilometer (3,100-mile) range, which can reach any corner of Europe within minutes.
The questions then arise:
* Will the West's successful deterrence of conventional attacks of the past 38 years hold under the new conditions?
Or might Moscow now calculate the West would never dare use its inferior theater nuclear weapons against superior Soviet theater nuclear weapons (especially when such use would clearly devastate the very West German territory being ''defended'')? Might the Kremlin then conclude it could use with impunity its superior conventional offense to steamroll NATO's inferior conventional defense?
* In the political corollary, might Moscow then conclude that its conventional and nuclear superiority in Europe could be used to force Western Europe to accommodate to Soviet foreign policy? This is the crux of the matter. Could we now reenter a period of Berlin crises every other month?
In autumn of 1983 the West's strategic thinkers suggest that the answers to these key questions reside in large part in the strength of NATO's conventional deterrence.
This is by now a truism for soldiers like General Rogers. ''We have mortgaged our defense to the nuclear response,'' he asserts time and again. ''The alliance , if attacked conventionally and if it is to defend [itself] successfully, will have to resort fairly quickly to its second response'' of nuclear weapons. He continues: ''I don't like [that] fact.'' And he advocates escape from it by building NATO's conventional capacity to frustrate any conventional Warsaw Pact attack.
Rogers's concern about NATO's nuclear addiction is based as much on political as on military grounds. Antinuclear protests and opinion polls alike demonstrate that significant numbers of Western citizens have come to view the West's new nuclear weapons more as a danger than as a protection - as making nuclear war more rather than less probable. In particular, significant numbers of West Europeans appear rather more frightened than the Soviets are by the new American Pershing II and cruise missiles that are to begin deployment in Europe in December.
In the short run - no matter how effective these weapons may be militarily - their scaring of the very people they are designed to protect seems self-defeating. And in the long run, such an anomaly cannot be sustained in democratic societies.
A strengthened conventional balance could offer the way out of this dilemma, the strategic planners maintain. It could bolster deterrence of a Soviet conventional attack, raise the nuclear threshold, and reassure anxious publics that a nuclear war is the least likely development in Europe.
A strengthened conventional balance could also offer a way out of the moral anguish inherent in basing defense on one's own initiation of use of nuclear weapons.
But is this a realistic hope?
That is what this series attempts to find out.