A border row dating back to the collapse of Yugoslavia is threatening Croatia's chances of completing EU membership talks this year and becoming the bloc's 28th member by 2011.
Slovenia, the first former Yugoslav nation to join the EU in 2004, has been blocking talks with Croatia because of the 18-year-old dispute, which mainly concerns a small bay in the Adriatic Sea.
Seen from a boat, Piran is a glorious sight - a Venetian-style city built of limestone and marble, lapped by the blue waters of the Adriatic.
It's easily missed on a map, but this small bay of just 20 square km (eight square miles) is making big diplomatic waves.
The row centres on access to international waters, which Slovenia treasures because its coastline is just 46km (29 miles) long. By contrast, Croatia's huge Adriatic coast stretches for 1,700km (1056 miles).
However, Croatia claims that the border should be drawn down the middle of the bay of Piran, which Slovenia fears would deny its ships direct passage to the high seas.
When both nations were part of Yugoslavia, borders didn't really matter.
But since they declared independence in 1991, incidents have flared up in the bay, with fishing boats seized and nets damaged.
"It's a big problem," said Milenko Bulezan, a fisherman for 20 years. "If we know where the border is, we go to that line, but we don't know now."
"We're afraid to go over this border," Mr Bulezan said. "The Adriatic Sea before was also for Slovenia, not just for the Croatian side."
Slovenia is smaller than Croatia, but it holds a powerful position. Like any EU member state, it can veto Croatia's bid.
Last December, it did exactly that, blocking a large chunk of its neighbour's accession talks.
The reason, according to Slovenian Foreign Minister Samuel Zbogar, was that Croatia had provided maps and documents in the EU negotiations that would have pre-judged a solution to the border dispute.
"We have problems allowing Croatia to continue negotiations, because our vital interests are still hurt by the documents that were brought into the EU process," Mr Zbogar says.
Time is running out for Croatia. Unless the stalemate is broken in the next few weeks, it is unlikely to complete EU membership talks by the end of the year.
But the two countries differ on the solution.
Slovenia prefers EU mediation, possibly led by the Nobel Peace Prize winner Martti Ahtisaari, the man who drew up the blueprint for Kosovo's independence.
Croatia says the border dispute is a legal issue, not a political one, and should be handled by the International Court of Justice in The Hague.
"We have one bilateral issue which is not related to our negotiation process," Croatian Foreign Minister Gordan Jandrokovic said. "And because of this issue we are now in the blockade."
"The message is that if we fulfil all that's necessary for membership, if we undertake some reforms that are very painful, there is no guarantee that we will become a member of the EU," he added.
Almost uniquely in the former Yugoslavia, Croatia and Slovenia have never gone to war with each other.
But the border dispute has now sparked what some call the Facebook wars.
Over 40,000 Croats have joined a group on the social networking site calling for a boycott of Slovenian products.
Outside a Slovenian-owned shopping mall in the Croatian capital Zagreb, several people said they didn't think they were getting fair treatment.
"I do not love Slovenians because they always act like we are enemies," one young mother told me. "They are small, they have their frustration about that, they want the Croatian coast, everything."
"We could live together like good neighbours, but from the beginning, Slovenians aren't acting like friends," she said.
Two hours' drive away, in the Slovenian capital Ljubljana, student Nejc Jemec showed me a Facebook group called "Slovenia + Croatia = Best Friends."
He joined it along with thousands of others to protest against another group, "Red Light to Croatia," which supports the government's tough line.
"I hope that things will get better in the future," Nejc said, "and that people will prepare not to think irrationally, but to admit that it would be probably easier for both of us if we were both members of the EU, it would be easier to solve problems."
A meeting between the prime ministers of Slovenia and Croatia later this month may lead to a solution. But no date has been set.
A complicating factor is the attempt by two Slovenian nationalist groups to hold a referendum on Croatia's Nato accession.
They say Croatia should be prevented from joining the alliance at a summit in April because of the border dispute.
A Nato spokesman voiced concern at the move, which would be unprecedented.
Macedonia's hopes to join Nato and the EU have also been dealt a serious blow by Greece, because of a row over the country's name, which Athens believes could lead to territorial claims.
As Serbia and other Balkan nations line up to join the Western clubs, there's growing concern that a dangerous precedent has been set.