The European Union has invited 10 mostly eastern European countries to join the bloc by May 2004. Half of them, plus two others, are scheduled to join the NATO alliance at about the same time.

Following its successful introduction of the Euro single currency at the beginning of 2002, the EU redrew its map by year's end to incorporate Estonia, Latvia, Lithuania, Poland, the Czech Republic, Slovakia, Hungary, Slovenia and the Mediterranean countries of Cyprus and Malta. But it stopped short of giving long-standing candidate Turkey a firm date to begin its own entry negotiations, saying such talks would begin soon after Turkey is deemed to meet EU criteria for membership at a December 2004 review.

The expansion, the biggest in the EU's history, will create a community of 450 million people in 25 countries and an economy of more than nine trillion dollars, close to that of the United States. Bulgaria and Romania are expected to join the EU in 2007.

The EU's expansion follows that of NATO barely a month earlier. The alliance invited Bulgaria and Romania to join, along with the three Baltic states, as well as Slovakia, and Slovenia. Poland, the Czech Republic and Hungary joined NATO in 1999.

The eastward moves by both organizations have reintegrated countries cut off from the European mainstream for more than six decades. They reverse the partition of the continent that was forged by Hitler's armies and solidified by Stalin's Iron Curtain.

In order to join NATO, the newcomers had to strengthen their democracies, subordinate their military to civilian authority and work out diplomatic solutions to long-standing nationalist quarrels. But as the alliance shifts the focus of its mission to fighting terrorism and containing rogue states, they, as well as their western European counterparts, will have to move aggressively to modernize their military forces to be able to close the capabilities gap with their American allies.

The EU, on the other hand, faces the challenge of adapting its institutions to accommodate its new members, most of them considerably poorer than the average western European country.

To qualify for membership, the newcomers had to adopt EU legislation. But, as analyst Steven Everts, of the Center for European Reform in London noted, there is some doubt as to whether they have the administrative capacity to implement EU laws.

"It's slightly easier to pass the legislation, but it's somewhat more difficult to be able to implement it. So there's a bit of a question mark. And the second question mark is over the levels of corruption in eastern Europe, which are still higher, on the whole, than they are in the present EU," Mr. Everts said.

Mr. Everts believes the EU will be exerting considerable pressure on the newcomers in the months ahead to make progress in those areas before they actually join the union. He said he worries more about the EU's ability to make decisions once it becomes a bloc of 25 nations.

"It's already very, very difficult for 15 countries to agree on what kind of environmental rules they want or, indeed, what kind of foreign policy position they should take vis-?-vis Iraq or other issues. That's already difficult with 15. It's going to be even more difficult at 25," Mr. Everts said.

Another analyst, Kirsty Hughes, of the Brussels-based Center for European Policy Studies, said the adoption by the EU Council of Governments of a system of weighted votes based on population to replace the old consensus rule on most issues will make the EU slightly less unwieldy. But she still has her doubts as to whether the EU can manage business and avoid paralysis in the future.

"It's decided how many votes in the council each country will have, how many members of the European Parliament each country will have, so that the most simple basic things to carry on functioning have been decided. But there is a much bigger question of what is this new EU going to be for. It's going to have 25 members. It's going to be diverse economically, geographically, politically. Is it really going to work? And I think that, partly, we can't answer that until we get the countries in and you start seeing the real political dynamics," Ms. Hughes said.

Much depends on whether a Convention on the Future of Europe, now meeting in Brussels, can reach a consensus on a radical overhaul of EU structures, and whether EU member states will accept its proposals or water them down in the ensuing inter-governmental negotiations.

In any case, the battle lines between those countries that want a more closely integrated union with members deciding key issues like taxation, immigration and foreign policy by majority voting, and those that want national governments to be able to veto such decisions, appear to be hardening.

One seasoned diplomatic observer says many of the newcomers, fresh from throwing off the Soviet yoke, are not likely to want to be caught in Brussels' stifling embrace. He thinks most of the new members see the EU as a union of nations that come together and cooperate. But there are also those who believe that, since most of the new members are small, they would support a federal EU to uphold the common interest and rein in the propensity of big countries to try to get their way.