The appeals chamber of the UN International Criminal Tribunal for the former Yugoslavia (ICTY) on Tuesday overturned the acquittal of Serbian strongman Slobodan Milosevic's secret police chiefs and ordered their retrial for war crimes.
Jovica Stanisic and his deputy Franko Simatovic, both 65, were immediately taken back into detention pending a new trial on all counts.
In May 2013, the ICTY acquitted the pair of war-crime charges - murder, forcible expulsions and persecution on ethnic or religious grounds - committed by police units they ordered into Croatia and Bosnia during the 1991-95 wars.
The aim of the terror campaign was to ethnically cleanse the non-Serb population from certain areas in the two countries, both of which split from the former Yugoslavia in the early 1990s.
The ICTY found in the first instance that Stanisic and Simatovic did order police units to Croatia and Bosnia, where they committed crimes, but said there was no proof that they were directly responsible for them.
Ruling on an appeal by the prosecution, which had asked for lifelong imprisonment in the first trial, the appeals chamber said that it found serious errors in the first-instance judgment.
The Stanisic-Simatovic trial is regarded as important because it could prove a direct link between Milosevic's regime in Belgrade and crimes of ethnic cleansing on the ground in Croatia and Bosnia.
Both countries filed genocide charges against Serbia, citing atrocities from the Yugoslav wars, at the UN International Court of Justice. Serbia was cleared in both cases.
Milosevic, whom the ICTY accused of genocide, died in detention in 2006 before the verdict in his trial was delivered.
The United Nations established the ICTY in 1993 to look into the major war crimes in the conflicts that erupted with the disintegration of Yugoslavia from 1991 until the last conflict, in Macedonia, a decade later.