What lessons can be learned from the experience of third-party mediation in a failed peace process? This study examines the unprecedented role played by a nongovernmental organization, the Henry Dunant Centre for Humanitarian Dialogue (HDC), in brokering negotiations and implementing an agreement reached between the Indonesian government and the armed Acehnese separatist movement GAM. The negotiations led in 2000 to a cease-fire, known as the “humanitarian pause,” that held fitfully into 2001. The major breakthrough, however, came in December 2002 when the parties signed the Cessation of Hostilities Agreement (COHA). This pact envisioned a cease-fire followed by demilitarization measures and an “all-inclusive dialogue” on autonomy provisions followed by provincial elections in Aceh. Within months, however, this agreement had broken down and Indonesian security forces had launched their largest-ever military operations in the restive province. Based on interviews and review of the literature on third-party facilitation and mediation roles in internal conflicts, this analysis tries to understand what caused the breakdown by looking at the strengths and weaknesses of a nongovernmental institution like HDC in mediating and then attempting to guide implementation of complex, risk-prone accords between battle-hardened adversaries. In Aceh’s case, was the conflict ripe for resolution? And if so, what could HDC, or any nongovernmental organization lacking formal power, do to bring about a successful accord? Analyzed in depth are the choices that HDC faced in its third-party role––and some possible lessons that might be learned about the sequencing of negotiation issues, dealing with fractious parties, and ensuring that a third party has sufficient political authority for peace implementation. This is the ninth publication in Policy Studies, a peer-reviewed East-West Center Washington series that presents scholarly analysis of key contemporary domestic and international political, economic, and strategic issues affecting Asia in a policy relevant manner.