Closing the Gap: Serbia and the Unfinished Map of the Three Seas

Serbia is the missing link

The Three Seas Initiative (3SI) was conceived to knit Central and Eastern Europe together along a north–south axis, connecting the Baltic, Adriatic, and Black Seas through cooperation on transport, energy, and digital infrastructure. Launched in 2016 by Croatia and Poland, it began with 12 members and has grown into one of Europe's most consequential frameworks for regional cooperation, now binding 13 EU member states into a market of roughly 120 million people with a combined GDP of about €3.5 trillion. At its eleventh summit, held in Dubrovnik on 28–29 April 2026, the Initiative deliberately turned outward — courting private capital, signing energy and AI agreements, and linking its corridors to the India–Middle East–Europe Economic Corridor and the trans-Caspian Middle Corridor, with delegations from the Gulf and India invited to the table. Connectivity, in effect, has become the currency of alignment.

Yet for all the corridors the Three Seas has drawn — uniting members from Poland and the Baltics down to Croatia, Bulgaria, and Greece, and now reaching non-EU neighbours such as Albania and Montenegro — one stretch of that map remains conspicuously blank, precisely where Serbia sits. Serbia is the missing link the Initiative needs to run continuously from the Baltic to the Adriatic and Black Seas. Its absence does not merely leave a country outside a club; it fractures the very corridors the forum has spent a decade trying to make continuous, and it undermines the initiative's own architecture across energy, transport, and digital domains.

An initiative built to counter fragmentation

The Three Seas remains, by design, an informal platform with no permanent secretariat, working through an annual presidential summit, a parallel Business Forum, and a dedicated investment vehicle, the Three Seas Initiative Investment Fund (3SIIF), launched in 2019. Its project pipeline is now valued at roughly €111 billion and has grown from 48 priority projects in 2018 to 143 by 2024, with around 40 percent of the budget secured. Behind the numbers lie two enduring purposes: reducing the infrastructure gap that separates Central Europe from its richer Western neighbours — a shortfall estimated at between €600 billion and €1 trillion — and blunting Russia's historical leverage over the region's energy supply.

That second purpose is producing results. Russia's share of EU gas imports fell from roughly 45 percent before the 2022 invasion of Ukraine to about 12 percent in 2025, as the bloc diversified toward LNG, Norwegian pipelines, and Azerbaijani supply. The clearest single milestone came a year earlier: in February 2025, Estonia, Latvia, and Lithuania disconnected from the Soviet-era BRELL grid and synchronised with continental Europe, creating new north–south electricity highways that bypass Russia entirely. Similar investments in roads, railways, bridges, and ports are projected to generate up to $500 billion for the thirteen member economies over the coming decade, in a region that has grown at roughly double the Western European average for ten years.

The Initiative's financial arm has matured alongside its politics. The U.S. International Development Finance Corporation committed up to $300 million to the 3SIIF, lifting the fund's capital commitment to roughly €1.3 billion, and the fund has since deployed more than €800 million across nine countries — from Bulgaria's Burgas port to Baltic data centres — at a reported return near 15 percent. Washington's engagement is not incidental. For the United States, the Three Seas has become a platform for energy exports and commercial diplomacy, a way to anchor Central Europe to the transatlantic economy while managing the wider competition between China and itself. The Initiative has also widened its circle, admitting Ukraine and Moldova (2023) and Albania and Montenegro (2025) as associated participants, and adding strategic partners including the United States, Germany, Japan, Türkiye, Spain, and — at Dubrovnik — Italy.

The land bridge left off the map

The gap is in the middle of the map itself. Serbia borders four Three Seas members — Hungary, Croatia, Romania, and Bulgaria — while two of the Initiative's newest associated participants, Montenegro and Albania, lie just beyond it. Nearly every overland route running south from the Pannonian plain toward the Adriatic and the Aegean must cross Serbian territory. Pan-European Corridor X, the rail-and-road spine threading from Salzburg and Graz through Ljubljana, Zagreb, and Belgrade down to Niš, Skopje, and Thessaloniki, runs directly through the Serbian capital. The Initiative's north–south axis already runs through Serbia in everything but name; Belgrade was, after all, once a scheduled stop on the Orient Express.

What makes the omission conspicuous is who is already inside the tent. The Dubrovnik Declaration of April 2026 committed the Initiative to anchoring Ukraine, Moldova, Albania, and Montenegro through a transparent, merit-based EU enlargement process. Yet Serbia — the largest economy in the Western Balkans and the literal land bridge between those associated states and the Central European core — appears nowhere on that roster. Two of Serbia's smaller neighbours have been welcomed into the framework without EU membership, while the country that physically connects them to it remains outside. The question is no longer whether Serbia belongs on the Three Seas map, but how long the West can afford to leave the keystone of its own connectivity architecture blank.

Transport: one spine, two chequebooks

The backbone of any southern extension is Corridor X. A train from Athens to Budapest routed through Serbia covers roughly 330 kilometres less than one sent around through Romania and Bulgaria; on the Initiative's map, Serbia is the direct route rather than the long way around. Modernising it would strengthen Serbia as a freight and logistics hub, deepen nearly $2 billion in annual trade with Croatia, and link the rail network more effectively to Adriatic ports such as Trieste and Bar. Much of the line is already being rebuilt — but with money from two very different sources.

To the north, the 350-kilometre Belgrade–Budapest high-speed railway is roughly 85 percent financed by China's Export-Import Bank, including a loan of about $1.3 billion to Belgrade. The Belgrade–Novi Sad section opened in March 2022 and carried 6.83 million passengers in its first two years; Novi Sad–Subotica followed in October 2025; and through-service to Budapest began in early 2026, making it China's most strategically significant infrastructure project completed inside the EU. To the south, the story is European: the 230-kilometre Belgrade–Niš upgrade rests on a €2.2 billion Team Europe package agreed in 2023 — up to €598 million in EU grants, a €1.1 billion EIB loan, and a €550 million EBRD loan — engineered for speeds up to 200 km/h. The same spine, in short, is being built with Chinese loans north of Belgrade and European money to the south. Serbia's network already carries the Three Seas' north–south traffic; what remains unsettled is whose standards and whose money the finished system will answer to.

The Danube adds a second dimension. Flowing through Serbia for nearly 600 kilometres, it is one of Europe's busiest inland waterways. Improving navigation, clearing shallow-water bottlenecks, and modernising ports would turn Serbia into a genuine multimodal hub where road, rail, and river transport intersect — while also supporting hydropower, electricity interconnections, and green logistics.

Energy: a diversification Serbia has already begun

For most of its modern history, Serbia bought gas from a single supplier along a single route — Russian gas arriving via the Balkan Stream extension of TurkStream. That changed at the end of 2023. In December 2023, Serbia commissioned a 170-kilometre interconnector from Niš through Dimitrovgrad to Sofia, with an annual capacity of 1.8 billion cubic metres — close to 60 percent of national consumption. The European Commission classed it a Project of Common Interest and helped fund it, with a €49.6 million grant and a €25 million EIB loan covering most of the Serbian section. Through the Bulgarian grid, the pipe connects to Azerbaijani gas via the Southern Gas Corridor and to LNG landing at the Greek terminal of Alexandroupolis, where Serbia has booked capacity; Belgrade has since signed a commercial contract with Azerbaijan for 400 million cubic metres a year. This is precisely the diversification logic the Three Seas exists to reward — carried out by a country without a seat at the table.

The energy story now extends to nuclear power. In late 2024, Serbia lifted a 35-year moratorium on nuclear construction, and in March 2026 France's EDF presented a roadmap for a national nuclear programme, with small modular reactors central to the plan. These bets are not separate from Serbia's digital ambitions but the condition for them: the supercomputers Belgrade wants to run cannot operate without the new baseload and balancing power now being planned. Small modular reactors, designed to sit beside the very data centres driving demand, would let the digital and energy pillars rise together rather than one waiting on the other.

The digital pillar doesn't wait for accession

It is on the digital front that Serbia has moved fastest. Its IT-services exports reached a record €4.55 billion in 2025 — roughly a tenfold increase since 2012 — and are now the country's single largest export industry. The government has adopted an Artificial Intelligence Strategy for 2025–2030 backed by more than $70 million, anchored by a state data centre in Kragujevac that houses an NVIDIA supercomputer, took delivery of a second in late 2025, and is meant to grow into the largest such cluster in Southeast Europe by 2027. Most telling for the Initiative: in October 2025 Serbia was selected as one of the EU's AI Factory Antennas, plugging it into the same EuroHPC supercomputing architecture as member states — EU accession or not. The digital corridor, in other words, has already crossed the border the political map has not.

Why Serbia matters

Serbia should not be viewed as simply another applicant. Transportation corridors, energy networks, and supply chains function best as one connected system, and Serbia's geography makes it the connector between Central Europe, the Western Balkans, the Danube basin, and the Eastern Mediterranean. Including it would reinforce Croatia's role as an Adriatic gateway, extend Poland's reach into Southeastern Europe, remove the most conspicuous bottleneck on the north–south axis, and align a strategically located non-member with Western partners on infrastructure resilience, cybersecurity, and dual-use networks — all without disturbing Serbia's long-standing military neutrality. Leaving it out invites others to fill the gap: the Belgrade–Budapest railway is already the Belt and Road's flagship inside Europe, and every corridor the West declines to finance in Serbia is one a rival is glad to build.

What Serbia, Brussels, and Washington should do

1. Arrive with a portfolio, not a request. Belgrade should establish a single coordinating office and publish a costed, investment-ready Serbia Connectivity Portfolio that maps every existing project onto the Initiative's pillars, written in the Three Seas' own reporting template. The cheapest signal of seriousness costs nothing but a decision: adopt the Initiative's technical standards — TEN-T rail specifications, the ETCS Level 2 signalling already going in on Corridor X, and EuroHPC digital norms — unilaterally, paying the price of entry before anyone asks for it.

2. Make every long-horizon energy bet an explicitly Western one. Pair the gas-interconnector network with Western nuclear vendors and pursue a U.S.–Serbia joint statement on civil nuclear cooperation and entry into Washington's FIRST programme for responsible SMR deployment. Doing so converts a hundred-year procurement choice into a hundred-year alignment, and positions cross-border balancing assets as exactly the kind of project the 3SIIF and the EIB exist to co-finance.

3. Get financed inside the framework without waiting for membership. The 3SIIF invests on commercial terms regardless of EU membership — it already owns Bulgarian, Polish, and Austrian assets, and Serbian projects can benefit from the same treatment. Serbia should ring-fence any cohesion-funded components for members while drawing its own sections on the €6 billion EU Growth Plan for the Western Balkans, the Western Balkans Investment Framework, and U.S. DFC capital. The boldest option reverses the supplicant posture entirely: a Serbian capital contribution to the fund, as Estonia made, would buy a seat at the investment table that no application could.

4. Turn participation into status, and clear objections on purpose. Serbia should apply for associated-participant status on the precedent Montenegro and Albania set, place companies such as EPS, Srbija Kargo, the Kragujevac operator, and Telekom Srbija inside the Business Council's sector networks, and seek an early observer seat on the new parliamentary track. Where Croatia and the Baltic states hold reservations, Serbia should answer them with deliverables rather than grievances — a standing mechanism for the Danube border-island dispute, firm minority guarantees, and joint projects that visibly remove Russian energy from the region.

5. Mount a joint U.S.–EU push to close the gap. The final step is not Serbia's to take alone. Washington and Brussels should treat Serbia's inclusion as a shared connectivity interest and back it through a trilateral compact binding the U.S. commercial-diplomacy agenda to the EU's Growth Plan and enlargement track, with the next annual summit as the deadline. Closing the gap is how the West keeps the Balkan core of its own connectivity project from being completed by someone else.

Closing reframe

None of this is simple. Full membership is reserved for EU states, and Serbia is not one; some members tie their support to its accession pace and its refusal to sanction Russia; and the Kosovo question hangs over every application. Yet each obstacle has a route around it — associated-participant status, the Business Council, the fund's commercial financing, the Western Balkans Growth Plan — rather than a closed door. What is left is a choice the West makes, not Serbia. The corridors are already being built: the rail, the pipelines, the reactors, and the data centres are rising whether or not the map acknowledges them. Inside the Initiative, they complete a line from the Baltic to the Adriatic and the Black Sea. Left outside, they become a gap that someone else will fill. As the Pupin Initiative argues, Serbia is not on the edge of the Three Seas Initiative — it is the missing piece that allows the broader network to function as intended.