A YEAR AGO, then-Secretary of State Colin L. Powell declared that the violence in the Darfur region of Sudan justified the term "genocide." That was the first time since the adoption of the U.N. Genocide Convention in 1948 that a government had accused a sitting counterpart of this worst of all humanitarian crimes, and Mr. Powell chose his words carefully. His language was based on a survey of Darfurian refugees commissioned by the State Department: Of 1,136 civilians interviewed, a third had heard racial epithets while being attacked, suggesting that the mass killings and evictions in Darfur constituted genocide. The survey also found that three in four refugees had seen government insignia on the uniforms of their attackers, leaving no doubt as to the guilt of Sudan's government.

Since that finding, the United States has led an international effort to end the genocide. The effort has not been as quick or decisive as the genocide finding warranted: The Bush administration's attempt to pressure Sudan's government with U.N. sanctions was feckless, and it made no effort to deploy a robust NATO peacekeeping force. But, little by little, American diplomacy has made headway. For now, the horror has abated.

The first success of Western diplomacy was to improve access to Darfur for humanitarian workers and supplies, ending the government's policy of systematically denying visas to aid officials and bottling up their equipment in customs. Next, Sudan's rulers were persuaded to accept the deployment of 7,700 African Union troops in Darfur, up from the handful who were there a year ago. Partly thanks to the presence of these troops, violence between government forces and rebels, and between government and civilians, has greatly diminished.

But the progress is incomplete and reversible. Fully 3.2 million people have been affected by the war, half of Darfur's population. Many of these subsist in crowded camps for displaced people, where they depend on Western charity. Although humanitarian access has improved since last year, it remains imperfect. In the Kalma camp, which is home to something like 160,000 displaced people, the government has refused to extend a Norwegian group's authority to coordinate the distribution of relief supplies and has imposed an economic blockade. Meanwhile low-level violence continues. Although the government has authorized the deployment of 7,700 A.U. troops, only 5,800 are in place so far -- a failure both of the African governments that had promised troops and of the Western governments that promised to support them logistically.

The progress over the past year demonstrates that the United States and its allies do have the power to save lives by the tens of thousands. It also suggests that, if the Bush administration had pushed harder and earlier, it could have saved many more people. This lesson must be remembered over the coming weeks and months. Outsiders need to persist in their efforts to broker peace negotiations between Sudan's government and Darfur's rebels; they must complete the deployment of the African Union force and continue to pressure the government for humanitarian access. The past year demonstrates that, if the United States and its allies pursue these goals with determination, they can get what they want. But if they lose interest in Darfur, violence may resume and humanitarian access may dry up. With so much of the population already displaced and weakened, Darfur's death rate could easily return to the horrific levels of a year ago.