Balkan EU neighbours clash on border deadline

That decision concerned an area in the Gulf of Piran, where the tip of Slovenia touches the Adriatic Sea alongside Croatia's much longer maritime border.

The court said two-thirds of the waters around Savudrija Bay in Croatia belonged to Slovenia.

It said Slovenia should have the use of a 10 nautical mile-long channel through Croatian waters to give it access to the international part of the Adriatic.

It also envisaged that Slovenian soldiers should leave the Sveta Gera mountain peak, which, under the ruling, now belonged to Croatia, and that the 100-or-so Slovenian citizens who owned properties along the now-Croatian coast should give them up.

But Croatia has said the ruling was invalid because The Hague tribunal had been "contaminated" in 2015 by a Slovenian judge on the arbitration panel.

Its allegation came after leaked tapes showed that the judge had shared confidential information with the Slovenian government.

The court acknowledged the breach, but said it was not serious enough to jeopardise proceedings.

Croatia then withdrew from the arbitration, but the process continued without its participation.

Talks fail

The leaders of both countries met in Zagreb on 19 December to try to resolve the issue.

But the talks ended in open disagreement, with Slovenia saying it would implement the ruling unilaterally and with Croatia saying it would not budge until the matter had been settled via new negotiations.

"Slovenia will start the implementation where it can do it alone," Slovenian prime minister Miro Cerar said at the time.

Croatian prime minister Andrei Plenkovic told press: "It [29 December] is a day like any other. Croatia doesn't feel bound by it … The issue of borders is an open issue for Croatia".

Croatia's Plenkovic also warned that unilateral implementation by its EU neighbour "could cause incidents on the borders", referring to the risk that each side's coastguards could try to enforce the rights of 50-or-so local fishermen to operate in the disputed waters.

EU intervention

The issue is now likely to end up in the EU court in Luxembourg.

But Slovenia is also calling for the European Commission to come down on its side, given that respect of the arbitration ruling was enshrined in Croatia's EU accession treaty in 2013, which Slovenia had threatened to veto unless Croatia fell in line.

"Non-implementation of the decision means violation of international and European law and we will resort to a lawsuit [in the EU court] if Croatia does not honour the decision," Slovenia's Cerar said on 19 December.

Slovenia's foreign minister Karl Erjavec repeated the threat on Wednesday, adding that the commission should take sides.

Tina Strafela, a spokeswoman for Slovenia's EU embassy, also told EUobserver on Tuesday: "We are entering a phase where the [EU] commission will have to get involved actively".

The Slovenian side, in a position paper emailed to this website, warned that, minor fishing clashes aside, the issue "could have serious implications for the stability of the Western Balkans region" and for "further EU enlargement".

Its paper said the arbitration proceeding was meant to be "a model of peaceful and legal resolution for similar cases" in the Balkans, recalling that border disputes had been the cause of the Balkan wars in the 1990s in what it called "not so distant … history".

Croatia currently has open border disputes with EU-aspirant countries Bosnia, Serbia, and Montenegro.

Kosovo has also questioned a border accord with Montenegro, with Slovenia saying that if Croatia gets away with ignoring The Hague tribunal it could "raise appetites in Kosovo to do the same".

Rue of law

The commission earlier this month initiated sanctions proceedings against Poland over allegations that it had undermined rule of law via its controversial judicial reforms.

Frans Timmermans, the commissioner in charge of the Polish file, also said in July that he was "of course, ready to facilitate the process" of solving the Croatia-Slovenia problem.

But it remains unclear what powers the EU has at its disposal to enforce international arbitration rulings.

"The European Commission cannot interfere and it is likely to say the two sides need to agree themselves," Ivica Masturko, Croatia's former ambassador to Slovenia told Total Croatia News, an online news agency, on Thursday.