When Belgians boast that their nation is at "the crossroads of Europe," they are putting a brave face on a fact that frequently has been tiresome. As Brussels celebrates its 100th birthday, it looks back on a history of occupations.

And in this century Belgium at "the crossroads" has been in the path of mechanized armies.

Because it is the headquarters for the European Community bureaucracy, Brussels thinks of itself as "the capital of Europe." But the most salient fact about Europe today is that it finds tolerable, even desirable, a condition that hitherto seemed barely possible: economic might combined with political and military impotence.

This was much on the minds of some people who attended a recent conference organized here by Georgetown University's Center for Strategic and International Studies, a conference marking NATO's 30th anniversary.

In 1949 the Soviet Union appealed as a model society to a significant number of Western Europeans. The suppression of East Germany (1953), Hungary (1956) and Czechoslovakia (1968 and, above all, the pen of Alexander Solzhenitsyn have changed that appeal. But while the West has been winning the competition of ideas, it has been forfeiting the arms race.

It is true, but not especially comforting, that the Soviet Union remains an essentially backward society, exporting raw materials and importing technology, failing in every field of competition except the military field. Neither is it comforting, although it is true, to say that there are no material reasons, only political reasons, wthy the West is losing only in a field where it cannot afford to lose, and is losing to a nation that is, as Raymond Aron says, "the most spectacular failure in history," a nation that fears, and cannot even feed, its people.

During this decade, security problems have been far from the front of Europe's mind. As Walter Lanqueur writes, only in Spain, Portugual and Greece, and then only when regimes were changing, have issues other than domestic economic questions been paramount.

Today, it is said, there is a new European sobriety about the Soviet threat. And people note that NATO is a contentious subject only in Greece, Turkey and Spain. But that may be because NATO is not yet doing the difficult things that must be done, particularly regarding theater nuclear forces, to redress the imbalance in Europe.

In 1949, Soviet power was conventional and continental. The Soviet threat was confined to NATO's central front. The Soviet navy was competent only for coastal protection. Today the Soviet threat is global, and the threat to NATO is especially great on the flanks, particularly on the disintegrating southern flank.

Some of NATO's vital interests lie outside NATO's geographical perimeter, in, for example, the Persian Gulf. A war involving NATO could begin beyond NATO's perimeter and a NATO war probably would be a world war. There is among European leaders perfunctory talk about the importance of the Third World, but the talk sounds especially idle when accompanied by an unwillingness to take the political risk of even talking about extending NATO's military concerns beyond the European continent.

It is said, with an elegant lassitude, that a crisis is just a period between two other crises. It is said, phlegmatically, that the recent history of Europe is a story of calamities that didn't happen, such as the victory of the Left that did not occur in the 1978 elections in France. But there is more to be said, and Aron, Europe's most distinguished public philosopher, has said it.

"Decadent" is the word Aron applies, with a mixture of censure and affection, to "the Western rump of Europe" which "has become one of the world's three or four centers of industrial prosperity but which is incapable of defending itself." If the future belongs, as some have said, not to "the warriors" but to "the producers," the producers of cars and wheat and poetry, then the future is ours. But what if the future belongs, as the past so often belonged, to the warriors?

Aron asks: "Is it not virtu , in Machiavelli's sense, the capacity for collective action and historic vitality, that now, as always, remains the ultimate cause of the fortune of nations and of their rise and fall?"

Of the Western nations' failure to prevent the fall of Constantinople in 1453, Gibbon wrote: "By some, the danger was considered as imaginary, by others as inevitable." The threat to Europe today is neither.