It is Day Five in one of the most realistic war games that Pentagon planners have ever run, and Soviet troops have just penetrated NATO's forward general defense positions here in West Germany.

The American military leaders watching the make-believe war unfold will soon learn that on Day 19 Western Europe's rear defenses will fall under the weight of the Warsaw Pact's conventional armor and artillery -- unless a nuclear counterattack by NATO shifts the course of the battle, and of history.

A separate game, codenamed WINTEX, has American and European officials meet every two years to simulate exactly what their nations would do if the request to fire nuclear artillery shells or missile warheads came from that overrun European battlefield. In almost all cases, the question winds up on the desk of one man, the president of the United States.

In one exercise, the senior military official playing the president decided quickly to go nuclear and ranged deftly up and down the "ladder of escalation" by mixing low-yield atomic artillery shells and increasingly powerful medium-range missiles for three days until he was satisfied that the attack had been halted. An official playing "Europe" noted in anger afterward that not once had "the president" moved toward the button that would have launched Titan or Minuteman rockets from their U.S. bases into the Soviet Union.

The only reliable figures ever published on such an exercise indicate that at least 1.5 million West Germans would have died in three days of "limited" atomic war. It came from Helmut Schmidt, now chancellor of West Germany, in a book he published in 1962.

Despite that gruesome total, the exercises tend to make NATO's "flexible response" doctrine sound like a pragmatic and smooth way of dealing with the unthinkable -- World War III. The highly classified Pentagon war game even has a trendy "lead-in," with the war erupting in Europe only after U.S. and Soviet forces have been engaged in a lower-level conflict for 60 days in the Persian Gulf.

But the wave of nuclear dread passing over Western Europe right now is challenging the plausibility of such war-gaming for battles in Europe, and of the doctrine that is supposedly the foundation of such exercises. That doctrine has been conceived around a gigantic hole that the Pentagon had to acknowledge reluctantly this month when Secretary of State Alexander M. Haig Jr. and Secretary of Defense Caspar Weinberger publicly disagreed on what options NATO planned for that moment of Day Five when the conventional defenses crack.

In trying to prove both men right, the Pentagon said that there was no precise plan that spelled out what NATO could do at that moment. Haig had said a nuclear warning shot was one option, but the Pentagon's modified demur suggested a grimmer truth that European NATO officials affirm flatly -- that nobody knows what will happen when a tactical nuclear weapon is exploded in combat for the first time, and that chaos is the most likely outcome.

The strategy that is being explained by Haig and Weinberger and which supposedly protects the West against the continuing Soviet military buildup has been developed largely in a vacuum, well away from the reality of an unreliable stockpile of nuclear warheads based in Europe and away from full consultations with the Europeans, senior European officials in NATO governments concede.

The 6,000 U.S. nuclear warheads in Europe "are like totem poles," a former State Department official says. "Some people worship them, some people are frightened by them. The problem is that, more and more, our allies rather than the Russians are frightened the most."

Descriptions of the classified history of how and why, beginning in the 1950s, these weapons were put in Europe strongly suggest that there has never been a coherent plan for their use -- just a belief that the next war with the Warsaw Pact would be nuclear. With that notion, and large amounts of nuclear materials, the U.S. military services each moved rapidly to procure nuclear weapons of all kinds to fight it in the 1950s. During the next decade, the stockpile of warheads continued, in topsy fashion, to grow.

Then-secretary of defense Robert S. McNamara decided in 1962 to impose a 7,000-warhead ceiling as a way of cutting off the Joint Chiefs of Staff from putting ever more warheads into Europe. McNamara made the number public four years later to rebut charges that he was cutting back on nuclear weapons in Europe, and since then the ceiling has taken on a symbolic importance that has no relationship to its military value.

NATO never gives a breakdown of the composition of this stockpile, which was quietly reduced by 1,000 warheads last year. But from the published and private estimates made by experts it would appear that roughly one-third of the warheads are contained in short-range artillery shells. One thousand or more are air-delivered bombs. The remainder is formed by antiaircraft missiles, antisubmarine devices, ground-launched missile warheads, landmines and even a few aging air-to-air missiles.

The pattern of dispersal also has to be taken into consideration by military planners trying to master this unwieldly arsenal. Today, according to congressional sources, one-third of the 6,000 stockpiled warheads in Western Europe are to be fired in combat by U.S. forces, while the remaining two-thirds are intended for launchers in the hands of other alliance members.

The Dutch Army, for example, has been assigned a battery of nuclear-capable eight-inch howitzers, stationed in West Germany. These guns and the smaller 155mm batteries controlled by other nations would be fired first in most of the war game scenarios played out by NATO, probably as a demonstration as Haig said in the statement that Weinberger disputed.

The 20-year-old nuclear warheads for the eight-inch Dutch guns are actually in storage compounds in the Netherlands and would have to be flown into the battlefield area by helicopter for use. They can fire only nine miles and will release a dangerously high nuclear yield, of one to 10 kilotons, for a weapon that is to explode so close to friendly troops. They lack modern safety devices that permit their complex inner workings to be destroyed if captured by the enemy on the battlefield or by terrorists in peace time.

Or consider the stockpile of Atomic Demolition Munitions, known popularly as landmines and referred to by American nuclear scientists who originally designed them as "a Hiroshima bomb in a can." Their mission is to blow up enormous amounts of earth and thus block the major invasion routes leading into Western Europe.

The problem is that West Germany, where the mines would be most useful, has refused to allow in peacetime the construction of the cement chambers into which the mines would have to be placed in the event of an invasion. The alternative appears to be to have troops rush out ahead of the invading forces and construct a suitable site for detonation of a nuclear device. Again, the risk of loss to the enemy, if the president does not authorize an immediate strike, is severe.

At this moment, more than 200 U.S.- and NATO-piloted fighter-bomber aircraft are sitting in cement shelters at the end of Western European runways loaded with nuclear bombs totaling for each plane more that 100 kilotons of explosive power, seven times more destructive than the weapon that devastated Hiroshima. The pilots are prepared to be airborne within 15 minutes and on their way to preset targets in the Soviet Union and/or Eastern Europe.

If an order came from higher headquarters to release the weapons for use, Americans would arm the bombs by applying an ever-changing code. But in practice exercises, those orders to the American units have failed at times to arrive when needed. This could become vital if, as is widely expected by NATO experts, the Soviet Union made the "quick reaction aircraft" a prime target for conventional airstrikes in the opening seconds of a European war.

"There is no plan," an American former Army commander says. "There is only a policy of non-use." The stockpile has remained at its size not for military reasons, but primarily for the political reasons of reassuring allies and providing potential bargaining chips.

When the Army removed its Honest John missile launchers in the early 1970s, Henry A. Kissinger, national security adviser, ordered the almost 1,000 warheads for that system kept in Europe in hopes they could be traded away in arms control negotiations. In fact, they were finally removed in 1980 as a result of a unilateral U.S. decision to balance the decision to deploy Pershing II and Tomahawk cruise missiles.

NATO European planners interviewed for this series do not believe that the Soviets, any more than NATO members, have any idea of how they would actually use the megaton rocket warheads they have targeted on Western Europe if the real war buttons were pushed. But the Soviets have a huge advantage, since they do not have to emulate the allies in constructing explanations and defense postures that square with domestic public opinion and increasingly worried allies.

When the idea of a ladder of escalation on which each rung could be chosen and controlled as part of a "flexible and controlled response" was fashioned as such an explanation in the early 1960s, it was backed at least by NATO's superiority in two crucial areas: a preponderance of tactical nuclear weapons actually deployed in Europe and the undisputed superiority of the U.S. intercontinental missile force.

Both factors no longer hold.

"If the Soviet Union, by external actions such as a massive buildup, can make NATO's threat to use nuclear weapons seem patently suicidal," wrote John Barry recently in The Times of London, "then the doctrine will lose internal credibility in that the people of Western Europe, lacking a kamikaze tradition, will simply think it too risky to retain."

In part to respond to the military problem described by Barry, the 14 nations that belong to the North Atlantic Treaty Organization's military structure decided on Dec. 12, 1979, to deploy 572 Pershing II and Tomahawk ground-launched cruise missiles. The missiles were conceived by the Carter administration and by the leaders of West Germany, Britain and Italy as reassuring Western Europeans who perceive a growing shadow of Soviet missile deployment.

Instead, these missiles have become the objects of an unprecedented debate that has moved the discussion of nuclear strategy and weapons out of the hands of the tiny nuclear priesthood of experts who monopolized decisions and discussion on these awesome forces and forced it into the public arena. The missile furor has brought new attention in particular to the task of the NATO commanders who watch over the nuclear arsenal.

A short while before President Reagan got himself entangled in the subject, Gen. Bernard Rogers, the American general who commands NATO under the title SACEUR (Supreme Allied Commander for Europe), sat in his office at Mons, Belgium, and explained his soldier's view of how that first nuclear blast in the current version of flexible response might happen.

"If war starts," he said, "suppose we are losing conventionally, and it appears like we are going to lose the cohesion of our defense. Then SACEUR goes to the political authorities and says, you must authorize me to use theater nuclear forces in order to get the Soviet Union to face up with either the uncertainty of going to the strategic exchange or withdrawing."

It is at this point that an agreed-upon strategy and plan of action should merge the interests of the U.S. and European halves of the alliance, and it is at this point that doctrine emerges as a substitute.

NATO headquarters at Mons is a three-story, prefabricated concrete office building slapped together in a hurry in 1966 when Charles de Gaulle kicked NATO out of France. NATO officials like that image of flimsiness, for it gives them an opening to tell visitors that it proves NATO's existence is for the purpose of deterrence, not combat. (A new, well-protected underground command center is, however, under construction next door.)

That image of impermanence masks the long-standing controversy between the United States and its allies on how to effect deterrence. The Europeans have traditionally held to a view of absolute or pure deterrence, going on the argument that the more unthinkable a nuclear war was made to be, the less likely it would be. America's early strategy of massive retaliation thus suited Europe. But listen to Rogers continue on his role, and you will hear some of the problems the Europeans are having these days with flexible response, which had already begun to be affected by Jimmy Carter's move toward a strategy for fighting a nuclear war.

"Then they authorize me to use the theater nuclear weapons -- I would say at that stage on non-Soviet Warsaw Pact soil and on Soviet soil, but certainly not on our own -- on military targets. We send that message, we get our response" from the Soviets.

"The response says, all right, we either continue conventional, and we've already sent the message we're going to use theater nuclears, or they respond with theater nuclears and maybe a massive," meaning a massive nuclear strike on the United States.

"But they have to make that decision," said the four-star general. "Are they prepared with the uncertainties in doing that, to have us respond with a strategic nuclear, which would be our next level of escalation? I can't believe the Soviet Union is any more anxious than we are to have that kind of a strategic exchange.

"I happen to be one who believes there would be a very quick escalation to the strategic level," Rogers said, after stressing the importance in any event of showing the Soviets that NATO is prepared to try to control the escalation.

Even the vocabulary troubles a European expert who works at NATO headquarters in Brussels and who complains in an interview of a "misperception" among Americans who see NATO's European-based rockets on a level somehow lower than "strategic." For a European who will be hit by them, this official said, all nuclear weapons are "strategic."

There are also basic conceptual differences between the United States and Europe and what the term "nuclear warfighting" entails. Reports of U.S. Army plans for fighting on an "integrated battlefield" in Europe that would mix conventional, nuclear and chemical warfare unnerved West European government officials -- particularly since they had never been briefed on it by the United States. West German Chancellor Schmidt has been assured that the integrated battlefield exists only as a "training document," a West German official noted.

"What we're really talking about here is an attempt to have limited-use nuclear weapons, and we want to ensure that our adversary understands that in a case like this it would be a restrained use and for specific targets, for specific time frames," Brig. Gen. Niles J. Fulwyler, commanding general of the U.S. Army Nuclear and Chemical Agency, told CBS-TV in a documentary on U.S. defense shown this summer. "I feel that it's very important at least to be able to try to have the limited use of nuclear war and have that option, rather than when we cross that threshold to go straight to general nuclear release, which is everything."

But, out of deference to European sensitivities, the how and when of the use of short-range battlefield nuclear weapons has never been spelled out in an alliance plan. European planners see these weapons as basically political and symbolic rather than functional, since their use would almost certainly wipe out Europe.

The levels of force necessary to control each rung of the "ladder" of atomic war -- that is, how many short-range artillery shells should be used before SACEUR should go to Pershings -- have also never been spelled out. And, moves to define or strengthen a "rung" -- for example, by adding the antitank neutron warhead or Pershing II/Tomahawk cruise -- can push NATO into stress.

The lack of full consultation contributed to the 1977 furor over the U.S. proposal to produce and deploy neutron artillery shells and Lance missile warheads. The new generation of nuclear weapons had been discussed only in passing by the Americans at NATO meetings. When the move to a system that had radiation as the major killing mechanism became public through press accounts in Washington, European political officials were unable to deal with the public uproar at home because they had not been fully briefed on the nature of the new weapons, and had no declassified material with which to counter public attacks.

Now, this has again become an acute dilemma for European leaders who for the first time since the 1960s are being called on to follow through on their decision for a major nuclear deployment -- to educate their publics in a reassuring way while trying to portray the new deterrent force to the Soviet Union as so dangerous that war has to be avoided.

"In a democracy that is filled with nukes up to its ears, you have to talk about the threat to Europe. But as a result people feel threatened," said Karl Kaiser, director of the Research Institute for the German Society for Foreign Affairs. For Germans, who are prohibited from having their own nuclear weapons, "to discuss the issue of nuclear weapons at all is dangerous" because it reminds Germans they are part "of a foreign club."