Dutch military chiefs are appearing in court in the Netherlands in a case brought by families of some victims of the Srebrenica massacre in 1995.
The massacre was Europe's worst atrocity since World War II
More than 7,000 men were killed by Bosnian Serb troops who overran the Muslim enclave during the Bosnian war.
Victims' relatives say Dutch UN peacekeepers in charge of protecting the area failed in their duty, and they are seeking compensation.
The Dutch government has accepted partial blame for the massacre.
The administration of former Prime Minister Wim Kok resigned in April 2002 after an independent report said the Dutch government and senior military officials had failed to prevent the massacre.
The survivors want to know who gave what orders in the run-up to Europe's worst atrocity since World War II, says the BBC's Geraldine Coughlan in The Hague.
The first witness on Thursday, a personnel officer identified as B Osterveen, said the Dutch UN battalion was unprepared for a Serb onslaught.
Asked what the troops had done to ready themselves, the Associated Press news agency reported that he replied: "We hadn't considered that."
The officials, including the former defence minister, will give evidence relating to allegations that the Netherlands did not share information that could have prevented the massacre.
Their testimony at the preliminary hearing will help The Hague District Court decide whether there are sufficient ground for the civil suit to come to trial.
Elizabeth Zegveld, head of the legal team representing the survivors and the victims' families, told the BBC that Dutch troops should have protected her clients.
"It was not the Serbs that were expected to do anything else at the time, they were really the Dutch troops at the time that were expected to protect my clients - or at least their family members," she said.
"They were in an enclave that was supposed to be a protected area, more so they were in a compound of the Dutch that was a kind of legally protected area.
"So I think the UN military, the Dutch first and foremost, that should have done something, were in a position to do something to protect these people."
The hearings are expected to highlight some important legal issues surrounding the UN's overall role in peacekeeping missions, our correspondent says.
Last year, the Bosnian Serb government admitted that Serb forces had carried out the massacre and promised to bring those responsible to justice.