The world vowed "never again" after the genocide in Rwanda and the atrocities in Srebrenica, Bosnia. Then came Darfur. Over the past four years, at least 200,000 people have been killed, 2.5 million driven from their homes, and mass rapes have been used as a weapon in a brutal campaign - supported by the Sudanese government - against civilians in Darfur. In On Our Watch, FRONTLINE asks why the United Nations and its members once again failed to stop the slaughter.

Word of the burgeoning crisis in Darfur first came to the newly appointed U.N. humanitarian coordinator for Sudan, Mukesh Kapila, in 2003. "A young woman in her late 20s perhaps, who had trekked all the way from Darfur, sat in my office," Kapila tells FRONTLINE. "And she told me her personal story of how not only had she herself been multiply raped but also that her sisters and her family had also been maltreated in that way, and that this had actually been done by soldiers and people dressed in military and paramilitary uniforms."

But when Kapila confronted the government in Khartoum, he was met only with denials. "We said it during that time, that he's blowing this out of proportion," Abdelmahmood Abdelhaleem, Sudan's ambassador to the United Nations tells FRONTLINE. "And many like Kapila -- the U.N. is full of Kapilas."

But Kapila was also having difficulty getting action from leaders at the United Nations. Sir Kieran Prendergast, the undersecretary-general for political affairs and second in command to Kofi Annan, was hesitant about raising the profile of Darfur for fear of upsetting the peace process between the Sudanese government and rebels in the south, who had fought a 21-year civil war.

"The argument was always that if you could conclude the Comprehensive Peace Agreement, then that would provide a model which would enable you to settle the political side of the Darfur problem," says Prendergast, "which of course was only part of the problem."

The peace negotiations would go on for another year. By then the worst of the killing would be over in Darfur. The timing was no accident, according to Kapila: "When I spoke to my friendly contacts in the Sudan government in Khartoum, they told me that yes, they were also delaying the North-South peace agreement because they wanted to -- and I quote -- 'have a lasting solution in Darfur' before they signed the North-South peace agreement and the international community forced them to stop."

When the issue came up at the U.N. Security Council, the Sudanese government was supported by its powerful ally, China. James Traub, author of The Best Intentions: Kofi Annan and the U.N. in the Era of American World Power, tells FRONTLINE: "What has enabled Sudan to be as truculent as it has been is knowing that they have either the support, or at least the willingness not to act against them, of neighboring African countries, of Islamic countries generally, and of Russia and China. And above all of China. That support is critical for them."

More than any other power, the United States pushed for meaningful action against Sudan, but its standing had been weakened in the eyes of many in the international community by the war in Iraq. Prendergast tells FRONTLINE: "You know that most of the Third World regard non-interference and internal affairs as holy writ. And partly that's because they think, 'Who next?' And partly it's because the actions of NATO in Kosovo and the coalition in Iraq have agitated them in that respect and made them feel that the question of who next is a live and vivid one where they either hang together or they'll be hanged separately."

As the United Nations was proving ineffective in stopping the killing, a grassroots movement to save Darfur was building. FRONTLINE travels with actress and UNICEF goodwill ambassador Mia Farrow to the refugee camps in eastern Chad on her seventh trip to the region. "My first trip into Darfur was in 2004," she says. "This is it for me. … [I]t has eclipsed everything else in my life."

China's economic interests remained a major obstacle to intervention in Darfur. But activists would finally find a way to get China's attention -- by targeting the 2008 Beijing Olympic Games. Activist Eric Reeves launched a campaign to shame the Chinese government by labeling the games the "Genocide Olympics." "The message to China is clear," Reeves tells FRONTLINE. "We will ensure that your hosting of the Olympics will go down in history along with the 1936 Olympics in Berlin as an occasion of international infamy."

Within months, the Security Council, with China's support and Sudanese acquiescence, authorized 26,000 U.N. and African Union troops to be deployed to Darfur by the end of 2007. But for many refugees living in displaced persons camps in Darfur and neighboring Chad, the protection had come too late.

Update: On Feb. 12, 2008, filmmaker Steven Spielberg withdrew as artistic advisor to the Beijing Olympics. Activists had pressured him to resign over China's economic and diplomatic support of the Sudanese government. "At this point, my time and energy must be spent not on Olympic ceremonies, but on doing all I can to help bring an end to the unspeakable crimes against humanity that continue to be committed in Darfur," Spielberg said in a statement.

The United Nations' slow response has raised serious questions about a new doctrine, conceived after Kosovo and Rwanda, that the international community has a "Responsibility to Protect" civilians from human rights abuses, even abuses at the hands of their own governments.

"This is a doctrine that is stillborn," Reeves tells FRONTLINE, "and there could not have been a clearer case than Darfur for the Responsibility to Protect. It was exactly the situation that those who devised this [doctrine] had in mind. And yet Darfur is only revealing our greater failure."