A former senior United States official called the region a new Cold War battlefield, and said that Brussels was too rigid with the ways it tried to keep people on the good behavior track, while the money is not as connected as it should be to reform goals.

The official, who asked for anonymity to preserve influence in the region, said that the countries reformed only when Brussels and Washington worked together to push leaders hard to break old habits of corruption, state capture, a politicized judiciary and Russian shell companies trying to take over key infrastructure and media.

But Europe is not eager to import more problems. “The argument is that only by taking in the Balkan states are we assured to strengthen stability,” said Norbert Rottgen, the chairman of the German Bundestag’s foreign affairs committee. “But is that true?”

“If we import fragile states into the E.U., we import fragility,” he added. “If we compromise on conditions, we let in fragile countries open to foreign influences, so we have to be tough on the entry requirements.”

The irony of history, Mr. Bildt mused, is that had Yugoslavia remained together, it almost surely would have been in the European Union by now, having been well ahead in 1990 of current members Romania and Bulgaria.

“If the wars of dissolution hadn’t happened, all of this area would have been an E.U. member,” he said. “The Balkans have always lived best when integrated into a wider framework, as necessary today as in the past, and the one available today is the European Union.”

Mr. Kupchan remains an optimist. “We know where this story will ultimately end, with all the former Yugoslav states integrated into the European Union,” he said. “But when?”