Twenty years on, the debate about the Rwandan genocide in which up to a million people lost their lives still reverberates. The question of why Rwandans did this to each other and why the world stood by remains bitterly contested.

What is not in doubt is that over a period of 100 days the most widespread horrific massacre of modern times took place as the victims perished, mostly killed by hand with machetes. And even as that history is still fought over, an even angrier, more contemporary dispute rages about the nature of today’s government that first took power in the genocide’s aftermath.

Has it brought reconciliation, stability and recovery? Or merely substituted a new Tutsi minority for the ruthless Hutu regime that preceded it? Rwanda, and these questions, fundamentally influenced my years at the top of the UN. Five years after the killings, I joined the UN as head of its development agency, UNDP. Helping President Paul Kagame and his government overcome the legacy of lost life, lost capacity and lost growth was a permanent priority.