The European Union opened its doors today to six new nations, expanding the list of active applicants for its vast zone of prosperity to encompass nearly all of Eastern and Central Europe.

The invitations to begin negotiations to join the EU went to Bulgaria, Romania, Slovakia, Latvia and Lithuania, plus Malta. The EU's 15 current members also made Turkey a candidate for its rich free-trade club, but more tentatively. Turkey, which would be the EU's second-most populous member after Germany, would not begin discussions until it makes progress in instituting democratic practices and protecting human rights.

In Ankara, Turkish Prime Minister Bulent Ecevit acknowledged that "there may be details that are hard for us to digest," but he called the invitation "a great success for Turkey." He spoke after meeting with EU foreign policy head Javier Solana, who had been dispatched to the Turkish capital to sell the deal--which represented a turnaround from the EU's flat refusal two years ago to even consider Turkey.

The EU commissioner for enlargement, Gunter Verheugen, said EU nations hoped to be ready to receive new members by the end of 2002, though officials say the first probably would not join until at least 2003.

"This is a historical step toward a united Europe," Verheugen said. "The Iron Curtain has been definitely removed and the period of uncertainty ends."

Today's invitations were another step toward a Europe that is becoming more united and, its members hope, more of a power on the world stage. Eleven EU countries already have a single currency, the euro, and all of them will finalize Saturday the design of their own defense force, a rapid-reaction contingent of some 60,000 troops expected to become operational in 2003.

Yet, in a sign the EU still is a collection of countries and not a federation, France and Britain continued feuding today over France's refusal to accept imports of British beef, despite an EU requirement that it do so. And Britain effectively vetoed the creation of what was to have been a Europe-wide withholding tax on savings income intended to block tax evasion, sending technicians back to the drawing board.

The EU is seen as a pathway to prosperity by the candidate countries. Membership in the world's largest free-trade zone means markets for products, subsidies for agriculture, aid for poor regions and the status of becoming a full participating member in Western economic society.

But EU membership carries a heavy price. So that the new member countries don't break the EU budget or cast a cloud on the club, the EU requires applicants to deregulate and privatize their businesses, modernize their justice systems and remain firmly committed to democratic principles. Joining the EU requires reforms much more extensive than those necessary for joining NATO, which Poland, Hungary and the Czech Republic did last spring.

Even for the candidates actively negotiating the terms of membership, accession will take years of careful scrutiny and painful reform. The EU will monitor the way Slovakia treats its Gypsy population, for instance, and how Romania handles its orphanages, and how Poland reshapes its outdated agriculture system, and how Bulgaria shuts down an antiquated nuclear reactor. Not until those and many more concerns are satisfied can the candidate countries become members.

And before that, the EU must drastically reform its institutions and procedures to prevent total gridlock. A similar reform effort failed at an Amsterdam meeting in mid-1997, and no progress has been made in the areas, such as moving toward majority voting instead of unanimous approval, that caused the failure that time. Talks will begin next year on those reforms with an eye to finishing at the end of 2000.

In addition to the nations invited today to begin negotiations to join the EU, active candidates for EU membership are the Czech Republic, Poland, Hungary, Estonia, Slovenia and Cyprus. That brings the total number of applicants to 13, including the qualified offer to Turkey.

If all the candidates ultimately join, the EU would expand to 28 members. Ten years after the fall of the Berlin Wall, in the Europe writ large today, only the countries of the Russian-led Commonwealth of Independent States, the countries of the Balkan region, and Iceland, Norway and Switzerland are not EU candidates or members.

The case of Turkey, which would become the EU's only predominantly Muslim nation, has been a thorny one for the EU. Greece led strong opposition to its candidacy in 1997, and today it was one of several countries insisting that Turkey allow territorial cases against it be taken to the European Court of Justice and that its candidacy be linked to finding a solution to the divided island of Cyprus, among other conditions. EU officials said their terms were not negotiable.

"It's not bargaining," said Tarja Halonen, foreign affairs minister of Finland, which holds the EU's rotating presidency. "It's important that there are no misunderstandings."

The United States, appreciating Turkey's role as a NATO ally, supports its EU candidacy.

The invitation to the second set of countries today marked a change in strategy for the EU. The union has been planning to admit new members since 1993. But originally the idea was to have a "first wave" of more advanced Eastern European countries--the Czech Republic, Poland, Hungary, Estonia and Slovenia; they were invited to start negotiating in late 1997, along with Cyprus.

The others were considered second-rank candidates. But as time passed, some of the second-rank nations began making economic reforms more sweeping than those in the first wave, while some of the front-runners began falling behind in modernization.

This year's Kosovo war, meanwhile, persuaded EU leaders that they needed to reach out to as many countries in Eastern and Central Europe as possible. The EU is taking an active role in the reconstruction of Kosovo and trying to create closer ties with countries in southeast Europe.