On Our Watch, the devastating hour on Darfur that Frontline airs next week, is a first-rate primer on the ugliest side of transnational inaction, a perfect storm of isolated tragedy and lucrative global alliances that show how all the promises made after Rwanda and Bosnia are meaningless when put to the test. Producer Neil Docherty makes explicit the four-year timeline of events — newfound oil wealth in Africa’s largest country; an Arab government that outsources the ethnic cleansing of mostly black inhabitants to armed militias called janjaweed (“Devils on Horseback”); Sudan’s protective partnership with oil-hungry China as a bulwark against U.N.-sponsored action — that has turned the murder of more than 200,000 people and displacement of 2.5 million into a dizzyingly awful chapter in human history. Tales of rape and murder from witnesses and victims will chill your blood, and the similarities between humanitarian coordinator Dr. Mukesh Kapila’s unsuccessful early efforts to spark action from the U.N. and Lieutenant General Romeo Dallaire’s well-documented helplessness in Rwanda will break your heart, while the account of just how easy it was for Sudan to ride out resolution after resolution in the U.N. will offend your sensibilities. Only when the well-utilized weapon of international shame was thrown into the arena by grass-roots activists — a campaign launched by English professor Eric Reeves to refer to the 2008 Beijing Olympic Games as “The Genocide Olympics” — was China spurred to back down at the Security Council and allow troops into Darfur this year. To call it a turning point, though, would somehow feel wrong, an injustice to the lives already lost to wait-and-see politics and oil madness. There’s still room to screw up, while a genocidal evil still swelters and shifts and adapts in that region. Late is late, and as On Our Watch makes clear, the world has already failed these people. Tuesday, Nov. 20, 9 p.m., KCET.