Abuja, Nigeria ? Thanks in part to last-minute U.S. diplomacy, two years of Darfur peace talks beset by setbacks and frustration ended with a signing ceremony between the government and the largest rebel faction Friday.

Now the hard part: ensuring that pledges to stop the fighting and begin rebuilding translate to an end to Darfur’s suffering. The key may be a robust U.N. peacekeeping force, which Sudan’s government has indicated it is willing to accept.

While the main Darfur rebel group signed the accord, two others rejected it, saying it did not go far enough to meet their demands for security and power-sharing guarantees and compensation for war victims. Optimism was muted by the two groups’ absence and by a history of failure to live up to agreements.

Members of the fractious rebel camp are united in accusing Sudan’s central government of neglecting their impoverished western region but divided because of leadership rivalries and differing approaches.

The peace deal was backed by the African Union, the United States, Britain, the European Union and the Arab League. It calls for a cease-fire, disarmament of militias linked to the government and accused of some of the war’s worst atrocities, the integration of thousands of rebel fighters into Sudan’s armed forces and a protection force for civilians in the immediate aftermath of the war.

At least 180,000 people have been killed and more than 2 million forced to flee their homes in what the United Nations has called one of the world’s worst humanitarian crises.