Bosnian Serb wartime leader Radovan Karadzic who was brought to court Tuesday after his arrest on a Belgrade bus in 2008, Karadzic, 67, is charged with masterminding the murder of nearly 8,000 Muslim men and boys by forces loyal to him in the eastern Bosnian enclave of Srebrenica in July 1995.
The massacre, when Bosnian Serb troops under the command of wartime general Ratko Mladic overran Dutch UN peacekeepers, was the worst atrocity committed on European soil since World War II. Over the space of a few days, thousands were systematically executed and dumped into mass graves in the area.
Karadzic's legal adviser Peter Robinson said his client would argue that "no policy was being implemented (at Srebrenica)," asserting that the former Bosnian Serb leader "did not know prisoners would be executed."
He added that Karadzic, who risks life imprisonment if convicted, would tell the judges that while he did not deny that people were killed in Srebrenica, he "challenges the scale of the massacre".
Prosecutors say Karadzic, former Yugoslav president Slobodan Milosevic and Mladic acted together to "cleanse" Bosnian Muslims and Croats from Bosnia's Serb-claimed territories after the collapse of Yugoslavia in 1991.
Milosevic died midway through his own trial for genocide and war crimes in March 2006.
Karadzic, a poet and trained psychiatrist, is also charged for his alleged role in the siege of the Bosnian capital Sarajevo between May 1992 and November 1995 in which 10,000 people died under terrifying sniper and artillery fire.
Like Mladic, he has also been charged for his alleged role in taking hostage UN observers and peacekeepers to use them as human shields during a NATO bombing campaign against Bosnian Serb targets in May and June 1995.
Indicted by the International Criminal Tribunal for the former Yugoslavia (ICTY) in 1995, Karadzic spent 13 years on the run before being arrested in 2008 in Belgrade where he practised as a doctor of alternative medicine.
His trial began in October 2009 and prosecutors put their case against him between April 2010 and May this year.
Judges dropped one genocide count in June, saying there was not enough evidence to substantiate the charge for killings by Bosnian Serb forces in Bosnian towns from March to December 1992.
Genocide, the gravest crime in international humanitarian law, is the hardest to prove.
Karadzic plans a four-hour statement to open his defence, followed by the testimony of Russian colonel Andrei Demurenko, the UN chief of staff in Sarajevo from January to December 1995.
Wives and relatives of victims of the massacre will look on from the public gallery.
Karadzic, who has been allotted 300 hours for his defence, has said he will call 300 witnesses to testify on his behalf.
The names include Greek President Carolos Papoulias, who was Athens' foreign minister during the Bosnian war.
Karadzic has said Papoulias' testimony could prove his innocence for the infamous shelling of Sarajevo's Markale market on February 5, 1994, in which 67 people died.
Tuesday is a historic day for the ICTY as it also sees the start of the trial of Croatian Serb rebel leader Goran Hadzic, the last of 161 war-crimes suspects to be handed over to the court.