RAGUE, Dec. 12 — No one questions that this is a historic moment: Not quite all of Europe, but most of it from the Atlantic Ocean to the Russian border, is all but certain to agree this week in Copenhagen to fulfill a decades-long dream and fuse itself into a single entity in the name of peace and prosperity.

The trouble is that few people — either in the 15 nations already in the European Union or in the 10 others being invited to join by 2004 — are entirely sure this is a good thing.

"It depends what you mean by good," said Charles Gati, professor of European studies at the School of Advanced International Studies at Johns Hopkins. "If your ideal objective is to see Europe as a united entity, that is good for peace and stability, then this is an extraordinary move."

"If you look at it from a short-term perspective," he added, "there will be serious problems."

The rubble from the Berlin Wall fell 13 years ago in huge piles of hope. But the reality is proving, as ever, more complicated. The current, relatively wealthy members of the European Union are facing fears of being overwhelmed by new members in the east that are far poorer, and that may send waves of immigrants westward, taking jobs and creating new pressures on economies that are, at the moment, far from robust.

While the leaders of the 10 prospective new members — most from the former Soviet bloc — largely support the union, many of their own people fear becoming second-class citizens in a club they have little control over. Joining the European Union is less a romantic aspiration of the excluded, but a hard-nosed evaluation of benefit versus cost.

In last-minute negotiations today, those tensions rose to the top as Poland, the largest of the prospective countries and a potential powerhouse in a new European Union, continued to hold out for more subsidies to its farmers and aid to its government.

Poland's bottom-line complaint is the same, merely louder, that many other candidate countries have: that what they call harsh requirements for entry essentially relegates them to a lower tier of Europe.

"The unusually difficult conditions dictated to us means that accession to the European Union, maybe not generally, but immediately, may be in doubt," said Lech Kaczynski, the newly elected mayor of the capital, Warsaw, who campaigned with a heavy helping of Euroskepticism.

Few experts believe that the summit meeting will fail, predicting that the final knots will be eased by Friday or Saturday. At the same time, though, the union's commissioner for enlargement, Gόnter Verheugen, warned today on German television that it was "now or never" for the planned expansion. "If we don't succeed now, it will become more difficult in the future," he said.

John Palmer, political director of the European Policy Center, a research organization in Brussels, said last-minute bumps always accompanied expansions of the union. It is not surprising, he said, given the huge ambitions of this project — to swallow in one gulp Poland, the Czech Republic, Slovakia, Hungary, Slovenia, Estonia, Latvia, Lithuania, Malta and Cyprus.

"I genuinely do think it's historic," he said. "This is the de facto unification of Europe under conditions of democracy and the rule of law. It does finally bury any prospect of war in Europe."

The 15 members of the European Union are preparing to take in 75 million more people, almost half of them in Poland. Many of those 10 countries have made enormous strides since then: Prague, the Czech capital, is now to the eye quintessentially Western European. Poland is busily expanding its roads and business culture. Tiny Slovenia is quietly and industriously pulling itself to the top of the heap, as the rest of the former Yugoslavia grapples with the dislocations of war and corruption.

Still, the countries are largely poor. The second richest, the Czech Republic, has an average gross domestic product of $8,900 per capita, less than a third of its neighbor, Germany. Slovakia, once half of Czechoslovakia, has a G.D.P. per head of only $4,900. Corruption remains endemic, Soviet-style bureaucracy crushing, the infrastructure lacking, commitment to Western-style democracy often questioned.

Part of the theory of accepting new members is that they will more quickly approach Western European standards — as Greece, Spain, Portugal and Ireland have done.

But more grandly there is the idea, half a century after the last terrible war in Europe, of expanding what union bureaucrats call the "zone" of security and prosperity and thus prevent another war.

The potential new members have a more layered view. On one hand, many felt cheated living under Communism for 40 years and see Europe as their rightful place. Some simply see no option other than the European Union.

"There is no alternative," said Elemer Hankiss, a political scientist in Hungary. "You can join Ukraine or Kazakhstan or the union. The union is not the Garden of Eden, but certainly it's a better option than the other one."

At the same time, many in Eastern Europe — ruled for centuries by Soviets, Austrians and Ottomans — fear another empire where the center of power is far away. Even the benefits of joining may not be felt for many years.

Many prospective members, and especially Poland, are also angry about what they see as strict conditions for joining, giving initially smaller subsidies to farmers compared to those already in the union and delays for seven years the right to move and work inside the union.

"You don't want countries getting in that are angry, or unnecessarily angry," said Milada Anna Vachudova, a professor at the University of North Carolina at Chapel Hill. "The population is going to be much more emotional and short-term about their views of E.U. membership."

The big test will come next year, when the applicant countries hold referendums on whether to join. Public opinion polls show most people support joining, but public education on the issue is low.

In all the internal European debate about the expansion, an often overlooked issue is how it will affect the United States — and many experts are slowly concluding that it is very much in America's interests. For one, they say, a Europe with so many more members will be better able to shoulder military obligations, like peacekeeping in the Balkans.

But it also has the potential for diluting power from Western Europe, which has largely grown more hostile to the United States, in favor of Eastern European countries that have long viewed America as their protector against Russia.

"There is concern in Central and Eastern Europe, too. about some American policies," Mr. Gati of Johns Hopkins said. "But America has more friends relatively speaking in Central and Eastern Europe today than in Germany and France.

"Therefore, these countries being in the E.U., and especially if they do something about pro-American Turkey — it is good for us," he added.