The UN war crimes tribunal in The Hague has convicted two former Yugoslav Army officers accused of massacring nearly 200 prisoners in Croatia in 1991.
Vukovar was attacked by Yugoslav troops and Serb militiamen
A third man was acquitted of involvement in the deaths of at least 194 people who were sheltering at a hospital in Vukovar town.
All three defendants denied charges of murder, torture and persecution.
Some 1,000 of the town's residents died during the fighting in November 1991 and another 5,000 were taken prisoner.
The three men had been charged in connection with the deaths of 264 people who were taken from the hospital, but the court ruled that only 194 of them could be proved to have been killed.
Croats regard the siege of Vukovar by the Serb-dominated Yugoslav Army as a key event in the war for independence.
Mile Mrksic was sentenced by the International Criminal Tribunal for the former Yugoslavia to 20 years' imprisonment for murder and torture.
The court found that Mrksic had withdrawn Yugoslav troops guarding the prisoners "with the consequence that the territorial defence and paramilitary groups were able to murder the prisoners," AFP news agency reports.
Another officer, Veselin Sljivancanin, was sentenced to five years' jail on charges of torture, but was acquitted on charges of extermination.
The third defendant, Miroslav Radic, was acquitted.
At the start of the war in Croatia, hundreds of refugees, hoping to be evacuated in the presence of international observers, gathered at the Vukovar hospital.
The three men were tried for one of the war's worst atrocities
Prosecutors say the Yugoslav Army handed 400 Croats and non-Serbs over to Serb rebel forces.
The victims were forced on to buses and taken to a remote location where they were killed and buried in mass graves, according to the indictment.
The men were also accused of trying to cover up the murders afterwards.
After more than seven years on the run, the three were arrested and transferred to the UN tribunal in 2002 and 2003.
In 2005, a court in Serbia gave 14 former Serb militiamen prison terms ranging from five to 20 years for their involvement in the massacre.