Raphael Lemkin died - ignored, frustrated, exhausted - almost as anonymously as his parents, who perished in the Holocaust. His success story is movingly chronicled in 'Watchers of the Sky.' Success it is - if unfinished and under threat - because it was his inventiveness , commitment and perseverance that ultimately led to the definition and classification of the crime of genocide, to agreement on a Genocide Convention and, ultimately, to the establishment of the International Criminal Court. For Lemkin knew an awful truth: that there is a universal human capacity to carry out harms of great magnitude, that no-one is safe. His Jewish Polish family had experience of pogroms, and he was sickened and outraged when he learned about the Turkish genocide of Armenians, about Roman persecution of Christians, about French persecution of Huguenots, about Mongol persecution of minorities. But he saw a pattern across the centuries and had a connection with all victims and was determined to use law to prevent it. He was shocked to realise that the killing of a million people was, in effect, less than the killing of a single man, because the law could not deal with it. He spent 50 years trying to make possible legal action against the crime of aggression against national, ethnic or religious groups. The Nuremberg trials after the Second World War proved the point, because the Nazi perpetrators could be prosecuted only in terms of war. But what about genocides that occurred within a national boundary, outside of war? Rwanda. Darfur. No wonder governments didn’t like the idea. It infringed on their “right” to do what they wanted within their own borders. Persuading governments was no easy task. For a start, he had to find a name for this vast crime. He came up with ‘genocide’. Then he set about getting the UN to pass a revolution, which he did partly by cleverly appealing to each country’s own experience of genocide at the hands of others. He had to overcome enormous scepticism by politicians, diplomats, journalists. So do his successors, some of whom are movingly portrayed in the film: Ben Ferencz, a prosecutor at Nuremberg; Emmanuel Uwurukundo, who survived shattering experiences in Rwanda and now works for the UN helping refugees who have fled organised violence in Darfur; Luis Moreno-Ocampo, the first chief prosecutor of the International Criminal Court who supervised the case against Sudan’s President Omar Al-Bashir; Samantha Power, an award-winning journalist who saw Ratko Mladic at work in the break-up of Yugoslavia and who took up law and diplomacy in order to play a more direct role in combatting future genocides. The documentary shows them at work and listens to their stories: the horrors they describe are well-known but their re-telling and the personal touches revealed – none more wrenching then Uwurukundo’s – are gripping. The truculent Bashir roams freely. The Criminal Court is accused of making it harder to achieve peace in some cases, or focussing on one region. Genocide has not been stopped: there will be others, and in many cases – as with Rwanda – outsiders will look the other way. But the battle is not lost. And though Lemkin did not achieve all he wanted, this lucid, compassionate, quiet, moving film – buttressed by beautiful animation and appropriate, non-intrusive music – is a fine tribute to Lemkin, his big idea, and all those who fight the cause. And the title? You'll have to wait til the end of the film to understand. By Daniel Nelson, One World · Watchers of the Sky is showing at the Human Rights Watch Film Festival in London on 25 March at the Curzon Soho, 6.15pm, and on 27 March at the Barbican, 6.30pm.