Answer
The answer is Aceh, a province in northern Sumatra, Indonesia. Ahtisaari played a crucial role in facilitating peace negotiations between the Indonesian government and the Free Aceh Movement (GAM) which had been engaged in a protracted armed conflict for decades. Through skillful mediation and tireless efforts, Ahtisaari helped bring an end to the violence and paved the way for a lasting peace agreement signed in 2005.