WESTERN Europe has now enjoyed, for two generations, its longest period of peace in this century. By next year it will have surpassed even the 1871-1914 hiatus between wars and enjoyed its longest peace in history. And this in a nuclear era that as never before threatens to make mankind extinct. This unprecedented peace, European governments believe, is no paradox of the nuclear age. It is instead a direct result of the unprecedented destructiveness of the atom bomb.
The very threat of nuclear war - and the risk that a conventional war might escalate uncontrollably into nuclear conflict - is so terrible that war as a whole is seen as suicidal and no longer a rational means of executing policy in those regions that are most vital to the superpowers.
Hence the advent of what has become known as nuclear ``deterrence,'' or prevention even of conventional war in Europe. This is the one spot on earth where Western policy calls for first use of nuclear weapons, should NATO forces face defeat by the Soviet bloc's superior conventional forces.
Yet now the American nuclear guarantee of Europe's security - and with it the robustness of deterrence of war - is being called into question in Europe. This is so for three reasons. First is a perceived trend toward ``denucleariza tion'' of Europe as Euromissile arms control begins a possible slide down the slippery slope of doing away with more nuclear weapons. Second and third are political and financial pressures in America to withdraw its GIs from Europe and reduce United States engagement there so as to cut costs, free America's hand for unilateral interventions elsewhere in the world - and, for some American politicians, teach European ingrates to pay for their own defense instead of freeloading off the US.
In Bonn, the conservatives' deputy parliamentary leader, Volker R"uhe, terms this the end of one strategic era and the beginning of another.
The profound uncertainty that this transition arouses is perhaps the major stimulus to today's resurrection of the moribund idea of joint European security.
The shock that Europeans felt in the wake of the superpower summit in Iceland last October and that they now feel from the Euromissile arms control deal that is in view is helping to some extent to drive West Germany, France, and Britain into more cooperative defense. The principle is the same one that drove the squabbling American states together 200 years ago: If you don't hang together, you are apt to hang separately.
This series, based on some two dozen interviews with senior diplomats, military officers, politicians, and academics in London, Paris, Bonn, and Brussels, explores the causes and prospects of the present jerky efforts to hang together.
Europe's nuclear predicament is a result of a number of factors like geography, the total vulnerability of all nations in the nuclear age, and the widely perceived insanity of any conceivable use of nuclear weapons. It also comes from the Soviet acquisition of nuclear superiority in the European theater in the early 1980s on top of Moscow's strategic parity of the '70s - and Soviet leader Mikhail Gorbachev's flexibility and his acknowledgment of the failure of the destructive potential of nuclear weapons to translate into specific political influence.
IN raw military terms, once the Soviets attained nuclear superiority in the European theater, NATO lost ``escalation dominance,'' or the ability to control the course of and limits on nuclear escalation.
This made irrational NATO's two-decade-old policy of ``flexible response'' to any Soviet attack by either conventional or nuclear weapons; both common sense and computer projections suggest that any NATO resort to nuclear weapons at this stage would only make things worse for the West, at a more destructive level.
That's all in the context of ``war-fighting,'' though. Aiming at deterring war (as distinct from actually fighting and winning it), NATO continued its policy of flexible response without change in the 1980s. And however much this might violate conventional logic, it made sense in the topsy-turvy nuclear world.
As David Abshire, former US ambassador to NATO, explains it, deterrence by ``uncertainty'' - if no longer by assured retaliation - is still very much alive. So long as the Soviets fear that in the passion and fog of war the West might commit the irrational act of nuclear escalation, this effectively deters any conventional attack, or even any attempt to convert Soviet conventional superiority into political intimidation.
The Europeans subscribed to this reasoning - until the Iceland summit and the Euromissile arms control deal now being negotiated by the superpowers revived European fears of Soviet-American condominium at the possible expense of Europe. The summit alarmed Europeans because of President Reagan's willingness, with no prior consultation with European allies, to contemplate scrapping all nuclear ballistic missiles, the very weapons that the Europeans regard as the most reliable guardians of their peace. Although British Prime Minister Margaret Thatcher helped get Mr. Reagan to bury the suggestion shortly after the summit, the fact that an American president could even float such an idea shocked the Europeans, especially the conservative bulwark of pro-American politicians there.
Suddenly the American Strategic Defense Initiative (``star wars'') no longer looked to its initial supporters in Europe like the welcome protection that would stiffen American backbone to risk Chicago in order to save Hamburg. It looked instead like the unwelcome protection that would turn the two superpowers into sanctuaries and leave Europe, and especially Germany, as the ``limited'' but razed nuclear battlefield.
Suddenly the Pentagon hard-liners no longer appeared to West German hard-liners as fellow crusaders against Soviet empire in Europe. Rather, they appeared to be unilateralists who wanted to write off an encumbering Europe.
AND suddenly the complaint of a Georgetown University strategist, Edward Luttwak, that the American lack of will to use nuclear weapons has already created a ``postnuclear'' world took on the aura of reality.
If the current Euromissile deal is clinched, NATO will rely more heavily on land-based nuclear missiles in the under-500-kilometer (under 300 miles) category. This projected shift to dependence on weapons that would devastate only Germany has fanned in Bonn the beginnings of a reluctance ever to use nuclear weapons. Within a surprisingly short time it became a common assumption in the West German center right that the Euromissile deal would ``decouple'' the US from Europe and invalidate flexible response.
Dismissing European fears, Washington contends that only a tiny part of the spectrum of nuclear weapons would be removed in any Euromissile arms control, and that this would not erode the still quite sturdy nuclear deterrence of conventional war in Europe.
French President Francois Mitterrand reasons along the same lines. So does Sir James Eberle, NATO's former commander in chief of the English Channel and present director of the Royal Institute of International Affairs in London. Admiral Eberle says he regards present arsenals as excessive in any case and views the ``complex theory of deterrence'' that has been ``woven'' around them as unnecessarily complicated. He challenges this view, saying:
``I have always entirely accepted [ex-Defense Secretary] Harold Brown's [thesis] that the likely result of limited nuclear exchange would be unlimited nuclear exchange.'' This, he suggests, scares the Russians as much as it scares the West.
But the notion that uncertainty is enough to deter hardly reassures governments that perceive the US as moving away from the deterrent threat of first use of nuclear weapons in Europe. West German parliamentarian R"uhe comments: ``I think the trend in the US is - and that is very serious - that nuclear weapons in future will only be used to deter nuclear use. That would be the end of NATO `first use' strategy. I think that is at the doorstep of [both] American parties.''
It's a ``historical trend, ... this philosophy to render nuclear weapons obsolete.''
John Roper, a former Labour member of Parliament in Britain and now a senior research fellow at the Royal Institute of International Affairs, concurs: ``Reduced emphasis on nuclear weapons is now the conventional wisdom in the United States.''
And even a serving official as circumspect as the British foreign secretary, Sir Geoffrey Howe, bluntly told the Belgian Institute of International Relations this spring: ``A distaste for reliance on nuclear weapons is not a new phenomenon in America. ... Some have long questioned whether the US would ever be prepared to use nuclear weapons in response to a Warsaw Pact conventional attack in Europe.''
Yet Europeans with a memory of Adolf Hitler's blitzkrieg attacks with inferior numbers of tanks in World War II fear that, without significant nuclear deterrence, even an unattainable conventional balance would be fragile and inadequate to ensure prevention of war. ``No one can conceive of conventional deterrence alone,'' asserts Mr. R"uhe.
NOR does the standard American assurance in recent weeks that US linkage to Europe rests not on ground-based missiles, but on its 326,000 troops committed to Europe, provide much comfort when Europeans see go-it-alone political and financial pressures building in the US. Former national-security advisers Henry Kissinger and Zbigniew Brzezinski have been leading the chorus to reduce American troops in Europe for some time, and American columnist William Safire's comments on European ``parochialism,'' miserliness in defense, and reluctance to help the US in Central America are familiar reading in Europe.
R"uhe warns, ``If there were a withdrawal of [US] troops on top of [the perceived trend toward denuclearization], it would really be disastrous and give the wrong signals.''
Roper adds, ``One of the things that is worrying is what may well be [US-European] strains in all three dimensions simultaneously,'' political and economic as well as military.
The former British parliamentarian also speaks of America's difficulties with the huge budget and trade deficits and the ``problem of both the trend to denuclearization [and] deemphasis on nuclear weapons in European defense, at a time in which anyone looking from now till the end of the century who predicts that there will be the same numbers of US troops in Europe [then] must be either a fool or a knave.''
All this requires some very fundamental rethinking within the alliance about what actually constitutes deterrence.
The questions include which mix of moral and strategic contradictions in nuclear policy is the least worst; what the conventional imbalance in Europe really requires in nuclear offset against a cautious Soviet evaluator of risks who is currently preoccupied with domestic affairs; and how support for Western nuclear policy can be maintained in a public that perceives a dwindling Soviet threat.
This period requires as well some fundamental rethinking about how the Europeans can maximize their contributions to defense in order to maintain stability and manage the pending reduction of American troops in Europe - without offering targets of opportunity to Moscow.
For a growing number of West German, French, and even British officials, part of the answer lies in renewing the quest for the kind of European defense cooperation that France so unceremoniously vetoed in the '50s by killing off the European Defense Community.
This doesn't come easily. But the urgency of Europe's present nuclear dilemma is certainly giving it fresh impetus.
One American diplomat notes the irony ``that we could be forcing the Europeans to take some of the tough decisions'' they would otherwise avoid.
In London, King's College Professor of war studies Lawrence Freedman, agrees that, unintentionally, the Reagan administration has been giving a boost to European cooperation by the anxieties it arouses. He does not expect dramatic steps forward.
But he concludes: ``I think it will be much more a process of drift and changes over time, so you suddenly realize 10 years on that it's all quite different. But if you ask when it happened, you can't quite figure it.
``I think this [evolution toward European cooperation] is happening, slowly. There is a greater readiness to recognize that you can't assume the security system of the last 20 or 30 years will continue'' this way forever.