The 100 years that just ended were widely regarded as the American century, as the United States rose to become the world's only superpower.

Will the next one belong to Europe?It may not look that way today, as Europe's economic growth lags far behind America's, its high-tech innovation falls short and its nations' governments act at cross-purposes in foreign affairs.

But the European Union, which includes some of the world's richest countries, and its much poorer neighbors to the east may have big surprises in store in the years ahead. Their joint project to enlarge the EU to as many as 30 member nations may well be the major event of the new century.

For the moment, the project to add the current list of 12 candidate nations to the existing EU 15 - enlarging the bloc's population and land mass by one-third - seems a distant dream.

Some of the candidates, such as Bulgaria and Romania, are fighting their way out of a deep economic pit. Even the most advanced of the ''first-wave'' candidates, such as Poland and Hungary, probably won't be admitted until 2005-2006.

Yet the EU's political leadership has signaled that enlarging the union is a top priority. Last month, the EU's Council of Ministers decided to add six countries (Bulgaria, Romania, Slovakia, Latvia, Lithuania and Malta) to the list of candidates, and said that Turkey also may become a serious candidate.

The council also decided to extend talks with the current front-runner applicants (Poland, Hungary, Czech Republic, Estonia, Slovenia and Cyprus) to encompass all 31 chapters of the ''acquis communautaire'' - the body of EU rules that new member states must adopt.

Through this plodding bureaucratic process, the EU's leaders hope to create the world's largest single market for trade and investment. The ultimate aim is to ensure security and prosperity throughout the Continent and perhaps achieve, through unity, more clout in world affairs.

Indeed, a successful enlargement would create an internal market of up to 480 million consumers, a lot bigger than the U.S. market.

While many of those consumers will be poor, they will live in rapidly growing economies with huge requirements for capital goods, raw materials and consumer products.

Traders and investors from both inside and outside the EU would benefit from a single set of rules, administrative procedures and external tariffs.

All this would create a scale for Europewide operations that would rival that of the United States. To take one example: The European rail network might, for the first time, reach the dimensions of the U.S. system, making possible economies of scale in cross-continental transport.

Up to a point, the diversity of growth levels and economic sophistication within an enlarged EU could be an asset. The flow of investment funds to Eastern Europe would intensify, as the logic of using the low-wage economies there as manufacturing and exporting bases becomes more compelling.

That, at least, is the optimistic scenario. There's also a pessimistic one: an EU unprepared for an influx of disadvantaged citizens, losing its focus and its core values as it struggles to give the newcomers a fair say without allowing them to swamp the bloc's institutions and deplete its treasury.

It's fear of the darker scenario that has kept the front-runner applicants waiting for almost the entire decade of the 1990s while the EU painstakingly negotiates the terms of entry and slowly moves toward getting its own house in order.

In particular, the EU needs to overhaul its Common Agricultural Policy, which already absorbs half of its budget and would bankrupt the EU if extended to the applicant states.

And it must restructure its central institutions, including changing the size of the European Commission and re-weighting votes in the Council of Ministers, to guard against decision-making gridlock.

The EU has experience with four waves of enlargement in the past quarter-century, but nothing quite like this one.

The prospective new members are poor, but then so were Ireland, Greece, Spain and Portugal. They are recovering from years of dictatorship, economic mismanagement and inward-looking policies, but then so were Greece, Spain and Portugal at the time of their accession.

It's the scope and diversity of the current enlargement - the bid to add 105 million people with historical frames of reference very different from those of the EU nations - that makes it daunting and promising at the same time.

That's why the path to enlargement is a vast paper trail consisting of endlessly detailed negotiations on standards, practices, rules and regulations.

Behind it all, however, is a political impulse - described by European Commission President Romano Prodi as the first chance to unify Europe since the fall of the Roman Empire.

Even if it avoids the nightmare scenario, an enlarged EU may fall short of that degree of cohesion and central power. But it could go a fair distance toward challenging American hegemony in the new century.