Six men, including ex-President Milan Milutinovic, face war crimes charges over the actions of Serb troops during the conflict in Kosovo in 1999.
The six deny murdering, persecuting and deporting ethnic Albanians.
The case has taken on a new importance after the death of ex-Yugoslav President Slobodan Milosevic during a trial on similar charges.
Opening the proceedings, prosecutor Thomas Hannis reminded the court of the stark images that preceded Nato military action in 1999.
"In 1999, Kosovo was prominent in international headlines and video images of conflict and convoys of thousands of Kosovo Albanian refugees," he said. We are going to explain why."
Mr Milutinovic is in the dock alongside former Yugoslav deputy Prime Minister Nikola Sainovic, ex-army chief of staff Dragoljub Ojdanic and three former generals - Nebojsa Pavkovic, Vladimir Lazarevic and security chief, Sreten Lukic.
They are charged with involvement in the murder, sexual assault and forced deportation of Kosovo Albanian civilians, as well as the destruction of their religious sites.
The trial aims to establish who was responsible for acting against Kosovo's Albanian community.
The six are accused of the forced deportation of 800,000 ethnic Albanian civilians and the murder of thousands of others in a "joint criminal enterprise" with Mr Milosevic.
"The evidence will show that these six accused were co-perpetrators with Slobodan Milosevic in a joint criminal enterprise... the aim of which was to ensure continued Serbian control over the province," Mr Hannis said.
The defendants, all of whom were appointed by Mr Milosevic, wanted to alter the ethnic balance in Kosovo, which was largely populated by Kosovo-Albanians, to perpetuate Serb control, he said.
He spoke of "widespread and systematic" murder, bombing, robbery and rape by Serbian forces across the province.
Mr Milosevic, widely seen as the architect of the Serbian activities in Kosovo, died in his cell in March, four years into his trial at The Hague.
His death means the tribunal has yet to establish legally what happened in Kosovo in 1999.
The prosecution says it needs at last a year to submit its evidence against the six defendants, and the defence is likely to take at least as long.Last Mod: 20 Eylül 2018, 18:16