Because of its nature and the seriousness of the issue, the letter detailing Slovenia’s intention to sue Croatia due to its refusal to accept the border arbitration award was personally delivered to the commission by Slovenia’s permanent representative to the EU, Janez Lenarčič.

The content of the letter is still confidential, but we do know that it mentions concrete violations of EU law, because of Croatia’s unwillingness to recognise the arbitration award, as well as all the other elements of the lawsuit.
In line with Article 259 of the Lisbon Treaty, once it receives the letter the EU Commission will have three months to respond and present its opinion. Before that it has to give both member states a chance to express their positions, either in writing or in person. The commission has until the middle of June to respond.

On the basis of the Slovenian letter, the commission can decide to sue Croatia, to give an opinion or give no opinion at all. But that does not prevent Slovenia from taking Croatia to the European Court of Justice in Luxembourg on its own.
In order to dispel any doubts about the legitimacy of the government, because of the PM’s decision to step down and the upcoming elections, on Wednesday the government adopted a decree stating that any new government that comes into office should go ahead with filing the lawsuit on behalf of Slovenia, in the event that the EU Commission does not file the lawsuit or if Croatia continues to reject the implementation of the arbitration award.
So far, 6 cases of one member state suing another have been concluded in Luxembourg. In only one case has the plaintiff been successful, and that was France in 1979. In all cases ending with a verdict, the commission was always on the side of the winner.
It is still unclear how the commission will respond this time. But there is a belief in Slovenia that the commission will not take its side.