SARAJEVO, Bosnia and Herzegovina — Cradle of the First World War, the Balkans have been a flash point, a place where empires, ethnicities and religions abut and contest. Now, analysts warn, the region is becoming a battleground in what feels like a new Cold War.

Russia, they say, is expanding its influence and magnifying ethnic tensions in countries that hope to join the European Union. Its involvement has already spurred Brussels to revive dormant aims for enlargement. It is also prompting fresh attention from Washington about security risks to NATO members.

After the concerted Western response to the poisoning in Britain of a former Russian spy and his daughter, expelling around 150 Russian diplomats and intelligence officers, “the Balkans become even more important,” said Mark Galeotti, a senior researcher at the Institute of International Relations in Prague.

“Russia is looking for ways to retaliate that are asymmetric and provide Moscow opportunities,” he said.

In a new paper for the European Council on Foreign Relations, Mr. Galeotti says that “Russia looks to the Balkans as a battlefield in its ‘political war,’ ” seeking “to create distractions and potential bargaining chips with the European Union.”

Charles A. Kupchan, who was Europe director of the National Security Council under President Barack Obama, said that “the Russians are taking advantage of the last part of Western Europe that remains politically dysfunctional.”

The situation bears distant echoes of Ukraine, where Russia originally agreed that Kiev could join the European Union — though not NATO — and then changed its mind, leading to the revolution that prompted Moscow to annex Crimea and foment secession in eastern Ukraine.

In the Balkans, the competition with Russia has the potential to sow fresh instability in a region still emerging from the vicious war of 1992-1995 that broke apart the former Yugoslavia.

In Sarajevo, many of the scars of have been erased. The former Holiday Inn, once a nearly windowless shelter for reporters near Snipers’ Alley during the Bosnia war, is restored and busy. The neo-Moorish City Hall, a monument to multiculturalism that was shelled and burned, has been burnished to a high standard.

Yet Bosnia and Herzegovina, the broken country patched together in 1995 at the end of the war, remains a fragile construct, riven by corruption, weak leadership and ethnic and nationalist strains among communities — a metaphor for the Balkans.

It is one of several key entry points that Russia is seeking to exploit, Mr. Kupchan said, as the leader of the Serb semiautonomous region known as Republika Srpska, Milorad Dodik, continues to press for an independence referendum. The others include Macedonia, where relations between ethnic Albanians and ethnic Slavs remain tense, and between Kosovo and Serbia.

Wary of Russian meddling, the European Union is holding out a renewed prospect of membership to Bosnia and to the other five nations of the Western Balkans — Serbia, Montenegro, Macedonia, Albania and Kosovo — in return for fundamental structural reform.

The skepticism among these countries about Brussels is deep, however. Many doubt the sincerity of a European Union that is turning more inward, more populist, more wary about migration and more cautious, after Romania and Bulgaria, about taking in nations before they are ready for membership.

No one believes any of these countries is yet ready to join. But the urgency for reform fell away as the goal receded.

Four years ago, the head of the European Commission, Jean-Claude Juncker, said that there would be no more quick enlargement of the bloc, sending the process into somnolence.

It has been, as the Macedonian foreign minister, Nikola Dimitrov, often says, like “being locked in a waiting room with no exit.”

“Juncker made a mistake to say that he was not interested in enlargement,” said Carl Bildt, a former Swedish foreign minister and United Nations special envoy to the Balkans. “The E.U. took its eye off the ball for several years, with detrimental effects.”

But with Britain leaving the bloc and Russia playing on the fissures of the region, the European Union has now laid out a relatively detailed plan for the Balkans.

It has even gone on record to say that if all goes well, Serbia and Montenegro, the only two countries now engaged in an accession process and hence the front-runners, could join by 2025.

The bloc’s strategy for the Western Balkans, published in February, laid out six initiatives: rule of law; security and migration; socio-economic development; transport and energy connectivity; digital agenda; and “reconciliation and good neighborly relations.”

Bulgaria, the current president of the bloc, will hold a special Balkans summit meeting in May, the Balkans are on the agenda for the European Council in June and the British will host a Western Balkans summit meeting in July, just before NATO has its own meeting in Brussels.

“It is time to finish the work of 1989,” said Johannes Hahn, the European Union commissioner in charge of enlargement. “We have set 2025 as an indicative date for Serbia and Montenegro, which is realistic but also very ambitious.”

Mr. Bildt said tartly: “Whether this is realistic or not remains to be seen.”

Many think it is too ambitious, given that the bloc insists that all these countries settle their numerous, passionate border disputes. There are also serious internal problems, the bloc’s report acknowledged.

“Today the countries show clear elements of state capture, including links with organized crime and corruption at all levels of government and administration, as well as a strong entanglement of public and private interests,” it said.

There is strong evidence of “extensive political interference in and control of the media” and lack of independence in the judiciary, it noted.

Add to that uncompetitive economies and the flight of young people looking for better jobs, and prospects seem dim.

But now the Americans are suddenly more interested, too. Renewed Washington concern “stems in part from concerns about expanded Russian influence,” said A. Ross Johnson, noting that Congress now demands that the Defense Department provide “an assessment of security cooperation between each Western Balkan country and the Russian Federation.”

Russia has made it clear that it considers new NATO expansion to the Western Balkans as unacceptable, and Moscow was implicated in a strange coup attempt in Montenegro in 2016 before that country joined NATO.

Russia is attempting to establish itself in the region, both with government and business, so that when these countries do finally enter the European Union, “they will bring Russian influence with them,” Mr. Galeotti said.

The strategy is similar to what China and Russia are doing with Greece and Cyprus, widely considered places where Russian money can be laundered into euros.

Russia is also deeply engaged in local language media, both with Kremlin-owned websites like Sputnik and with bots that harp on local grievances.

Mr. Bildt points in particular to Russian investment in key Serbian infrastructure, like energy. Though Russian investment pales compared to that of the European Union countries, Serbia has a natural affinity to its Russian Orthodox brethren and remembers Russian support during the Kosovo war.

“Is the E.U. sensitive enough to what is happening in Serbia?” Mr. Galeotti asks.

He thinks not. “E.U. policy has generally been to support whatever keeps the Western Balkans quiet,” Mr. Galeotti said. “It’s deeply dangerous and creates the perfect environment for Moscow to play its games.”

Brussels, he and others say, should put more weight behind both the carrots and the sticks — offering genuine incentives for institutional reform and accession, and genuine sanctions if not.

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 tries 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 Rӧttgen, 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?”