When EU leaders pose for a "family picture" with counterparts from six western Balkan nations hoping to join the bloc, Spanish Prime Minister Mariano Rajoy will stay away in protest - highlighting how long and hard their road to membership is likely to be.
Thursday's summit, the first such meeting in 15 years, is meant to demonstrate the European Union's renewed commitment to a region that remains fragile two decades after the ethnic wars that followed the break-up of Yugoslavia.
Spain does not even recognize the independence of Kosovo, which will attend the Sofia summit along with Albania, Bosnia, Serbia, Montenegro and Macedonia, and EU governments also worry about a string of other problems afflicting the region.
But after years of neglecting the six, the EU has been spurred into action by the growing influence of other powers in the region, which in 2015-16 also became a main route for a wave of migrants from the Middle East and Africa heading to wealthier European nations to the north.
EU chairman Donald Tusk made this point before the summit.
"It will be an opportunity for both sides to reaffirm that the European perspective remains the Western Balkans' geostrategic choice," he said. "I hope to bring our Western Balkan friends closer to the EU.”
As Britain is on the way out, the bloc's executive European Commission has proposed that EU leaders decide in June to open formal membership negotiations with Albania and Macedonia.
"The risks to Europe are zero," said Prime Minister Boyko Borissov of Bulgaria, which itself joined the EU in 2007 with neighboring Romania.
"If we do not embrace... the Western Balkans and do not help them - yes, many of them are not ready and they have yet to catch up - then there is no reason to be angry that the influence of the United States, Russia, Saudi Arabia will be greater than that of Europe," Borissov added.
But many in the EU feel differently.
As the bloc is still recovering from economic and migration crises that have fuelled euro-skepticism among its own voters, doubters point to problems ranging from organized crime in Albania to Macedonia's dispute with EU member Greece over its name, which is blocking Skopje's aspirations.
Rajoy has decided to leave Sofia before the western Balkans meeting and EU officials say no one from the Spanish delegation will pose for the symbolic joint photograph on Thursday - a reminder that Madrid is just one of five member states that do not regard Kosovo as a sovereign nation.
Madrid, locked in a dispute with Catalan separatists at home, refuses to recognize Kosovo's split from Serbia in 2008.
Timing in doubt
Two ex-Yugoslav republics, Slovenia and Croatia, have already joined the EU. But lawlessness and crime flourished in the Balkans during the wars of the 1990s, leaving the region awash with weapons and a transit route for drug and human traffickers.
"The direction of travel is very clear - the European perspective," a senior EU official said. "What really matters is the determination of applicants in implementing reforms. And patience because also on the EU side you need to have the right window of opportunity to take the decision.”
The EU and Balkan six will sign a declaration on improving infrastructure including electricity and gas connections, as well as countering radicalism, improving security and controlling migration.
Brussels also sees building good neighborly relations as vital to a region where wartime hostilities still burden relations and threaten the fragile peace.
To join the EU, Serbia - the biggest market among the six - must settle its borders with Kosovo and with Bosnia, where tensions between rival communities often paralyze decision-making.
Macedonian Prime Minister Zoran Zaev warned that a clear perspective was needed to prevent the region from sliding back into conflict.
"We saw that the status quo brings an erosion of democracy, a lack of economic opportunity," he told a meeting in Sofia.
"Negative influence from third parties is increasing.”