Following the breakup of Yugoslavia in 1991, Slovenia and Croatia became independent countries. As the border between the countries had not been determined in detail prior to independence, several parts of the border were disputed, both on land and at the sea, namely in the Gulf of Piran.
According to the Croatian Bureau of Statistics, the two countries share about of border. According to the Statistical Office of the Republic of Slovenia, the border spans . The border mostly runs along a southwest-northeast axis.
The countries have attempted to resolve the dispute, most notably with the Drnovšek–Račan agreement in 2001 that was ratified by Slovenia but not by Croatia. Because of the disputed border, Slovenia blocked Croatia's EU accession talks until the agreement was reached by both countries and the EU to settle the dispute by a binding arbitration.
On 29 June 2017, the Permanent Court of Arbitration issued a binding ruling on the border, ruling on the disputed parts of land border, drawing the border in the Gulf of Piran, and ruling that Slovenia should have direct access to international waters in the north Adriatic Sea using a corridor crossing Croatian waters. It also ruled on several other disputed border areas. The ruling was hailed by Slovenia but Croatia said it would not implement it. Croatia stated that they withdrew from the process in 2015, citing the discovered talks between the Slovenian government representative and the member of the arbitration court as a breach of the arbitration rules. Slovenia implemented the ruling on 29 December 2017 with continued opposition from Croatia.
Origins of Gulf of Piran issues
Following World War II, the area from north of Trieste to the Mirna River in the south was part of the Free Territory of Trieste. In 1954, the Territory was dissolved; the area was provisionally divided between Italy and Yugoslavia, and the division made final by the Treaty of Osimo in 1975.
In the first draft delimitation proposal following both countries' 1991 declarations of independence, Slovenia proposed establishing the border in the Gulf of Piran's center. However, Slovenia changed the draft the following year (declaring its sovereignty over the entire Gulf on 5 June 1992). Since then Slovenia has continued to insist on this position.
The name "Bay of Savudrija" () was originally used for only part of the bay. In 2000 it came into use for the whole bay by local Croatian fishermen and was quickly adopted (first by Croatian journalists, then local authorities, and finally at the state level), leading to its appearance in official maps. Such actions were contrary to established practices with long-standing geographical names, and are seen by Slovene authorities as an attempt to imply historical connections with the bay. Another name, "Bay of Dragonja" (), was introduced in Croatia but failed to gain widespread use.
Sea dispute
Croatia claims that the boundary should be an equal distance from each shore. The claim is based on the first sentence of Article 15 of the United Nations Convention on the Law of the Sea:
Slovenian claims are based on the same article. However, it favours the second sentence, which stipulates that historical claims or other unusual circumstances supersede the equidistance rule. The Law of the Sea also states that international waters begin from the country's shore – while the nearest international waters are away from Slovenia's shore.
The Convention states that a coastal state has an exclusive right to manage all natural resources in a band up to from its shore (an 'exclusive economic zone').
When Slovenia notified the UN Secretariat in 1995 of its succession to Yugoslavia's ratification of the Convention (continuing the agreement), it included a note saying that this system of exclusive economic zones has become part of international law and asserted its rights as a geographically disadvantaged state. The convention makes it clear that any decision to declare an exclusive economic zone should be made in co-operation with all interested parties; Croatian sources claim that Slovenia's self-description as a geographically disadvantaged state amounts to an admission that it is a country without access to international waters.