The 27-year-old, identified only as Jennifer W. in line with privacy rules, is charged with murder, a war crime and membership in a terrorist organization, and could face life in prison if convicted in Munich state court.
Pleas are not entered in the German system, and as her trial opened her attorney Seda Basay-Yildiz said the defendant had no statement to make to the court about the charges against her, the dpa news agency reported.
The girl's mother is to be called as a witness, and is also a co-plaintiff in the case as allowed under German law. Her lawyers include Amal Clooney, who specializes in international law and human rights cases.
Neither was in court for the opening hearing before the trial was adjourned for three weeks, but Clooney said in a statement it was hoped the case would encourage more prosecutions of returning Islamic State members for international crimes.
The United Nations has called the IS assault on the Yazidis' ancestral homeland in northern Iraq in 2014 a genocide, saying the Yazidis' "400,000-strong community had all been displaced, captured or killed." Of the thousands captured by IS, boys were forced to fight for the extremists, men were executed if they didn't convert to Islam — and often executed in any case — and women and girls were sold into slavery.
"Yazidi victims of genocide have waited far too long for their day in court," Clooney said. "I hope that this will be the first of many trials that will finally bring ISIS to justice in line with international law."
The defendant in the Munich case, who quit school after completing eighth grade, grew up in Lower Saxony as a Protestant but converted to Islam in 2013, according to a lengthy profile in the Sueddeutsche Zeitung newspaper.
Prosecutors allege she then made her way to Iraq through Turkey and Syria in 2014 to join the Islamic State. In 2015, as a member of the extremist group's "morality police," she patrolled parks in Fallujah and Mosul, armed with an assault rifle and a pistol as well as an explosive vest and looking for women who did not conform with its strict codes of behavior and dress, prosecutors said.
It was in the summer of that year that she and her husband, an Islamic State fighter, purchased the young Yazidi girl as a slave. One day when the girl wet her bed, prosecutors allege her husband chained the girl outside in the heat of the day and W. did nothing to prevent her dying of thirst.
The victim's mother was held with her in slavery by the Islamic State couple, and plans to testify about how they both were treated, another of her attorneys, Natalie von Wistinghausen, told dpa. "She wants justice," von Wistinghausen said. "She wants to have the opportunity to explain the fate that befell her and her daughter."
The defendant was taken into custody when trying to renew her identity papers at the German embassy in Ankara in 2016, and deported back to Germany. It was not clear whether she was investigated at the time, but prosecutors said she was arrested again in 2018 while trying to make her way back to Syria, and has been in custody since then. She has her own young daughter, but her attorney would not give any details about her, other than to say she's living with her grandmother in Germany and is "doing well."
The Sueddeutsche Zeitung reported her husband is believed to still be alive, living in the Turkey-Iraq border region.