During the brutal rule of Habre, who has been referred to as Africa's Pinochet, some 1,200 (according to Human Rights Watch) to 40,000 (according to a Chadian commission) Chadians were killed and 12,000 tortured (according to Human Rights Watch) by a network of secret police known as the DDS (Direction de la Documentation et de la Sécurité).
Chad itself never sought Habre's extradition, but he was sentenced to death in absentia in 2008 for his alleged role in a rebellion in the east of Chad that year. If he is found guilty in Senegal, he faces up to a life sentence.
The trial is a historic event for the continent: it is the first time one African country has prosecuted the former leader of another country for alleged human rights crimes.
(Clarification: Chad is not in ECOWAS, but Senegal, where the election will be held, is.)