Arbitration Commission of the Peace Conference on Yugoslavia
On 20 November 1991 Lord Carrington asked: "Can the internal boundaries between Croatia and Serbia and between Bosnia and Herzegovina and Serbia be regarded as frontiers in terms of public international law?" Applying the principle of "uti possidetis juris", the commission concluded on 11 January 1992, "The boundaries between Croatia and Serbia, between Bosnia and Herzegovina and Serbia, and possibly other adjacent independent states may not be altered except by agreement freely arrived at... Except where otherwise agreed, the former boundaries become frontiers protected by international law".
Slavonia
In the 1980s the political situation in Yugoslavia deteriorated with national tension fanned by the 1986 Serbian SANU Memorandum and the 1989 coups in Vojvodina, Kosovo and Montenegro. In January 1990, the Communist Party fragmented along national lines, with the Croatian faction demanding a looser federation. In the same year, the first multi-party elections were held in Croatia, with Franjo Tuđman's win raising nationalist tensions further. The Serbs in Croatia, intent on achieving independence from Croatia, left the Sabor and declared the autonomy of areas that would soon become the unrecognized Republic of Serbian Krajina (RSK). As tensions rose, Croatia declared independence in June 1991; however the declaration came into effect on 8 October 1991. Tensions escalated into the Croatian War of Independence when the Yugoslav National Army and various Serb paramilitaries attacked Croatia. By the end of 1991, a high intensity war fought along a wide front reduced Croatia to controlling about two-thirds of its territory.
In Slavonia, the first armed conflicts were clashes in Pakrac, and Borovo Selo near Vukovar. Western Slavonia was occupied in August 1991, following an advance by the Yugoslav forces north from Banja Luka across the Sava River. This was partially pushed back by the Croatian Army in operations named Otkos 10, and Orkan 91, which established a front line around Okučani and south of Pakrac that would hold virtually unchanged for more than three years until Operation Flash in May 1995. Armed conflict in the eastern Slavonia, culminating in the Battle of Vukovar and a subsequent massacre, also included heavy fighting and the successful defence of Osijek and Vinkovci. The front line stabilized and a ceasefire was agreed to on 2 January 1992, coming into force the next day. After the ceasefire, United Nations Protection Force was deployed to the occupied areas, but intermittent artillery and rocket attacks, launched from Serb-held areas of Bosnia, continued in several areas of Slavonia, especially in Slavonski Brod and Županja. The war effectively ended in 1995 with Croatia achieving a decisive victory over the RSK in August 1995. The remaining occupied areas—eastern Slavonia—were restored to Croatia pursuant to the Erdut Agreement of November 1995, with the process concluded in mid-January 1998.