Scenario: The war in Lebanon flares up again. Israel sends planes over the Bekaa Valley to wipe out Syrian surface-to-air missiles. Some Soviet advisers are killed. Moscow warns that it will not brook Western interference in Lebanese affairs.
Tensions rise. The Warsaw Pact begins unannounced maneuvers and calls up reserves. NATO goes on simple alert. By the time the Warsaw Pact attacks, NATO's 26 divisions are dug in near the East-West German border. The line holds.
Scenario: The war in Lebanon flares. Tensions rise - but there is no unusual military activity in Eastern Europe. Then, suddenly, the Warsaw Pact invades the north German plain from a ''standing start.''
Soviet SU-24 jets catch the bulk of NATO's aircraft on the ground - and with them half of NATO's total firepower. A Soviet operational maneuver group quickly assembles and drives a spearhead through the thin NATO line all the way to Hannover.
What makes the difference between these two scenarios is one vital factor: whether or not NATO is prepared.
If NATO troops mobilize in time to have the 1 to 4 days needed to move to their forward positions and dig in, the experts generally agree, the line holds, at least for a week or two.
If not, it doesn't. And if the line doesn't hold, that's the end for NATO, since West Germany's 150-mile east-west width (and Paris's removal of France as NATO's hinterland in 1966) leaves no room to trade space for time.
NATO's preparedness is the first and most crucial uncertainty in the European military balance. And it is followed by a host of other uncertainties in the complicated mix of armor and antiarmor, reconnaissance and targeting, electronics, counterelectronics, and counter-counterelectronics - and initiative and esprit. The arbitrary assumptions one starts with critically affect the conclusions one ends with.
So great are the complexities that a veteran NATO diplomat counsels that nobody really knows how a war in Europe would come out. ''The brutal fact of the matter is that real analysis of relative capabilities at theater level is a terribly imperfect art,'' the diplomat said. ''Opinions will start to vary quite widely depending on who is looking at it.''
And Anthony Cordesman, an editor of the Armed Forces Journal, sees the essence of the European military comparison as a ''balance of uncertainty.''
It is in this context that the two main schools of thought about NATO's conventional defense should be viewed.
The pessimists start with the beancount. They believe the Warsaw Pact's massive and growing quantitative superiority in weapons makes Europe insecure and unstable. ''The overall growing aerial-air defense imbalance has compounded NATO inferiority in armor, anti-armor, and fire support capability so much that the outlook is increasingly bleak for Western conventional defense,'' writes Phillip Karber, vice-president of BDM Corporation and former director of a long-term Pentagon study of the weapons balance in Europe.
The optimists, dismissing such static analysis as unsophisticated, retort that the many uncertainties in the European balance basically work in favor of the status quo, inhibiting any Soviet attack and enhancing NATO's defense. ''It disappoints me to hear people talk about the overwhelming Soviet conventional military strength. We can defend the borders of Western Europe with what we have ,'' asserted Gen. Frederick Kroesen in March 1983 after four years as commander of the US Army in Europe.
The 1983 study sponsored by the American Academy of Arts and Sciences, ''Strengthening Conventional Deterrence in Europe,'' is not that categorical in its optimism. But it does argue (1) that NATO defenses are impressive, (2) that Moscow must therefore be highly uncertain that any invasion of Western Europe would succeed, and (3) that this effectively discourages any attack by a Kremlin that is ultracautious in risk-taking.
This situation can be preserved, the study suggests, if NATO now makes prudent improvements in its forces and readiness.
More important - since analysts see the real threat not as a bald Soviet attack, but rather a Soviet brandishing of military might as a means of political intimidation - the low Kremlin confidence in the success of any military attack means that the Kremlin cannot now blackmail Europe.
This concept is a fundamental one. It follows largely from the Soviet political and military need for a quick victory in any European clash of arms.
Politically, the Soviet need to avoid a drawn-out war is obvious. The Kremlin strongly suspects - especially after the 1980-81 events in Poland - that the East Europeans who constitute half of Warsaw Pact forces would turn against Moscow in any protracted war.
Indeed, there are indications that the Kremlin doubts that it could fully ward off sabotage of its critical lines of communication across Poland to East Germany even in a short war.
Militarily, there are also compelling reasons for the Soviets to want a quick victory. Negatively, the shorter a war, the greater the chances of averting escalation to nuclear weapons. Positively, a surprise attack that succeeded in puncturing that thin NATO line could conceivably yield great rewards in the swift occupation of the heartland of West Germany.
In line with this thinking, Soviet forces are not designed for a long war effort. Their ''tooth-to-tail'' ratio of combat to logistics troops is very high (especially by contrast to NATO forces). They are deliberately structured for a blitzkrieg (''lightning war''), in which mobile, heavily armored, operational maneuver groups are expected to break through the enemy front line on the first or second day of battle, maintain an unprecedented offensive momentum of 40 to 50 kilometers (25 to 30 miles) a day, and quickly destroy nuclear sites, command centers, and communications nodes deep in NATO's rear.
If, in fact, the Soviets could accomplish such breakthroughs, the results would be disastrous for NATO. But Western optimists look at the flip side of the Soviet fixation on a blitzkrieg.
NATO does not need to winm a European war, they point out; it needs only to block that rapid Soviet victory and turn the intended blitzkrieg into a nasty war of attrition.
To state the argument in terms of deterrence, NATO does not need to convince the Kremlin that Moscow would lose a European war. NATO needs only to demonstrate that all the uncertainties make it unthinkable that the Warsaw Pact would win and win fast.
If the very conservative Soviet leaders are so persuaded, they are unlikely to undertake a highly risky attack. They therefore cannot brandish an implicit threat of attack to bend Europe to their will - and they have gained precious little in either added security or political utility by their dramatic military buildup of the past 20 years.
(This line of reasoning, of course, leaves out any scenario in which East-West confrontation has become so acute that war seems inevitable and one side launches a preemptive attack to gain the advantage of surprise over the other. The primary responsibility for avoiding such a situation lies with diplomacy, however, not with military structures.)
In this context the key question is not how various optimistic or pessimistic analysts in the West view the ''balance of uncertainty,'' but how the Kremlin views it. And here the best guide for anyone without access to Soviet General Staff meetings is probably the British Staff College's Christopher Donnelly, a man who colleagues say eats, breathes, and sleeps Soviet military tactics.
In the Soviet perception, Donnelly writes in this year's American Academy study, NATO has ''massive military potential,m far outweighing that of the Warsaw Pact.''
It has superior military technology and a geostrategic position of Britain and the US that could ''make [Soviet] victory in the Central Region inconclusive , and make it difficult for the Soviets to bring a war to the necessary speedy conclusion.''
If the Soviet Union is to avoid having these NATO strengths turned against it , it cannot settle for initial advances that then get bogged down. It must bring about ''collapse of the NATO political structure within a matter of days. . . . An invasion can stop at nothing less than the occupation of West Germany, the Low Countries, and the Baltic littoral.''
''If a war is to be won at all, it must be started suddenly and won quickly, '' while preventing ''escalation of the war to a nuclear holocaust. This is a tall order.''
Winning quickly is precisely what political scientist John Mearsheimer of the University of Chicago thinks the Soviets cannot do. He contends - if NATO mobilizes within a week of Warsaw Pact mobilization - that the Soviets cannot achieve the necessary force ratios on their main axes of advance to drive into NATO's rear.
To prove this case, Mearsheimer's new book, ''Conventional Deterrence,'' explores possible scenarios ranging from multipronged attack to concentration of Soviet forces at one, two, or three points on the inter-German border.
In the book's first alternative, the 57 1/3 Warsaw Pact divisions (and total manpower of about 1.25 million) face 28 1/3 NATO divisions (and total manpower of about 1 million) the length of the whole central front.
The ratios of the two sides stay approximately the same, Mearsheimer says, if NATO mobilizes as fast as the Warsaw Pact does. In manpower the ratio could approach 2 to 1 if NATO mobilizes a few days after the pact, but would then drop back to 1.2 to 1 as Western mobilization proceeds.
By the Soviet Union's own lights, Mearsheimer notes, these ratios do not offer a sufficient superiority to warrant an attack. A. A. Sidorenko's ''The Offensive: a Soviet View'' says that the lessons of World War II show a ''decisive superiority'' for an offensive to be ''3 to 5 times for infantry, 6 to 8 times for artillery, 3 to 4 times for tanks and self-propelled artillery, and 5 to 10 times for aircraft.''
Mearsheimer points out that the Soviets can certainly never get superiority of this magnitude with multiple (6 to 10) axes of advance. To achieve breakthroughs they would have to concentrate armor massively on one, two, or at most, three major axes of advance.
Terrain decrees that such probes would probably come at the Fulda Gap cutting across West Germany's wasp waist to the communications heart of Frankfurt only 60 miles from the East German border; the Gottingen corridor to the industrial Ruhr; across the north German plain to Hannover; and (least probably) the Hof corridor through Bavaria toward Stuttgart.
The Fulda, Gottingen, and Hof approaches all run through narrow corridors in wooded mountains - excellent country for defense and bad country for tanks. Even the north German plain, Mearsheimer contends, is no pushover for large armored divisions. There the obstacles consist of urban sprawl that provides ready cover for small-unit harassment.
Especially given the terrain and the defenders' familiarity with it, Mearsheimer believes, NATO forces can basically hold out. He finds a ''standing start'' attack almost as uncomfortable for unreinforced Warsaw Pact troops as for unreinforced NATO troops. And even a Warsaw Pact concentration on selected pressure points of 4 (or 5) to 1 superiority in manpower shouldn't prove overwhelming, he maintains.
This last deduction Mearsheimer draws primarily from ''force-to-space'' ratios. As he explains it, ''simply put, there is not enough room for the attacker to place all of his 24 divisions at the point of attack. He must therefore locate a portion of his divisions in subsequent echelons behind the attacking forces. . . . In essence, the defender is in the enviable position of being able to deal with the attacker's forces on a piecemeal basis.''
Mearsheimer further questions whether Soviet divisions haven't become so heavy that they are slowing themselves down - a clear handicap in any blitzkrieg.
Karber concedes much of this - ifm NATO forces are fully prepared to meet an attack. But NATO readiness at the moment of attack is precisely what he doubts. His nightmare is a Warsaw Pact attack on an unmobilized Western alliance of sovereign partners who squabble and vacillate in an ambiguous situation before ordering troops forward.
The Warsaw Pact's quantitative advantage, he contends in an interview, in conjunction with the pact's recently acquired ''forward-deployed posture in strength and force ratio'' could allow a ''short-warning'' attack.
''When you get a brigade in a peacetime casern [garrison] and it goes to the front into position, it does not take on the traditional advantage of defense until it puts in mines, cuts trees, has alternative firing positions so it can run up and fire and fall back.'' And before it has done this, the attacking ''force required to punch through a [light] screen is [only] 2 to 1. . . .
''I'm not saying they would do it. But it's an option we have to worry about. I would say in a crisis it's far less stable [now] than in the mid-'60s.''
At the other end of the time spectrum, MIT's William W. Kaufmann - an adviser to two decades of both Republican and Democratic defense secretaries - worries most about the European balance at 90 or 120 days after mobilization. ''I don't believe in short-warning cases,'' he says.
By contrast, however, by 90 days after mobilization the Warsaw Pact ''could not only get those 90 divisions in place, but could shape them [the undermanned and underequipped Category 2 and 3 divisions from the Soviet Union's western military districts] up to [full-strength] Category 1.'' And NATO simply would not be able to match these reinforcements.
After weighing the various ''grim'' trends against NATO's residual ''great deterrent strength,'' Cordesman concludes judiciously that ''the balance of uncertainty . . . does not support either fear or complacency.'' If NATO moves to correct its weaknesses, he says, ''NATO can unquestionably sustain enough conventional capacity to make a Warsaw Pact attack virtually unthinkable in the Soviet Union.''
In other words, NATO is not yet in serious trouble. But if it doesn't act soon, it may be. This is a formulation that both optimist and pessimist may agree on.