Kofi Annan, the first U.N. secretary general from sub-Saharan Africa, died Saturday after a short illness. Annan, a career U.N. diplomat, served as secretary general from 1997 to 2006 and received the Nobel Peace Prize in 2001 for his work toward “a better organized and a more peaceful world,” along with his contributions to the fights against AIDS and international terrorism.

But Annan’s long career as a peacemaker has a dark page: the 1994 genocide of Rwanda’s Tutsi minority. Annan oversaw U.N. peacekeeping efforts during this time, and the situation in Rwanda unraveled quickly. The inability of the United Nations to stop the genocide has been well documented, and Annan has been criticized for not heeding the warnings of violence.

There were clear signs of the violence to come

Annan was named head of the U.N. Department of Peacekeeping Operations in March 1993. Just five months later, he took on the task of setting up the U.N. Assistance Mission for Rwanda (UNAMIR) to oversee the Arusha Peace Accords between the government of Rwanda and the rebel Rwandan Patriotic Front (RPF), which had been at war since 1990.

But even as UNAMIR troops deployed in Rwanda, the situation in the country deteriorated. Signs that ethnic violence was imminent emerged as early as November 1993. As Alison Des Forges wrote in “Leave None to Tell the Story” (a report that I helped research and write as head of the Human Rights Watch office in Rwanda in 1995-1996):

The preparations for violence took place in full view of a U.N. peacekeeping force. The commander of that force reported evidence of the worsening situation to his superiors who directed him to observe the narrowest possible interpretation of his mandate. He was in effect to do nothing but keep on talking with the authorities while they kept on preparing for slaughter.

The primary superior to whom the UNAMIR commander, Canadian Gen. Roméo Dallaire, reported was Annan. Between January and March 1994, Dallaire repeatedly briefed Annan and other U.N. officials about the deteriorating situation in Rwanda, including a warning as early as Jan. 11 about a plan for “Tutsi extermination.” He repeatedly sought authorization from Annan to confiscate arms and otherwise act to prevent imminent violence, but was rebuffed.

Annan responded to Dallaire’s Jan. 11 cable by telling him that the request “clearly goes beyond the mandate entrusted to UNAMIR” and that “the overriding consideration is the need to avoid entering into a course of action that might lead to the use of force and unanticipated repercussions.”

Widespread killings began in April 1994

When Rwandan President Juvénal Habyarimana was assassinated by a missile that took down his plane on April 6, 1994, a cabal of powerful Hutu ethno-nationalists used the death as an excuse to launch a plan to wipe out their opponents and consolidate power. Just hours after Habyarimana’s death, elite presidential guard troops spread out into Kigali, the capital, to kill opposition politicians, journalists and civil society activists. Many of those initially killed were members of the Hutu ethnic majority, but prominent Tutsi were also targeted, and the killing swiftly took on an ethnic character.

Although the source of the missile remains unclear even today, the Rwandan government blamed the RPF. Because the RPF was a rebel army composed primarily of Tutsi refugees from earlier waves of ethnic violence, Hutu nationalists accused Tutsi in Rwanda of being agents of the RPF. Over the next several weeks, the genocide’s leaders systematically spread violence against the Tutsi minority into every community in Rwanda.

In his memoir, “Shake Hands with the Devil,” Dallaire criticized Annan for denying his requests to act more aggressively to prevent the genocide but also for forbidding UNAMIR from taking stronger action once the killing had begun. Annan refused to authorize UNAMIR troops to use force to protect threatened Tutsi, seize arms or detain those organizing the violence.

President Bill Clinton’s administration famously intervened to reduce the numbers of UNAMIR troops, over the strenuous objections of Dallaire (as detailed in Samantha Power’s “A Problem From Hell”). In the end, the genocide killed 500,000 to 800,000 Rwandans, overwhelmingly members of the Tutsi minority.

The U.N. and U.S. failed to act

Although the primary responsibility for the 1994 Rwandan genocide clearly lies with Rwanda’s government at the time and Rwandan leaders such as Col. Théoneste Bagasora, the actions of Annan, Clinton and other international leaders allowed the genocide to be much more lethal than it might otherwise have been. Annan did not act, despite warnings that violence was imminent.

Once the violence began, the U.N. decision to reduce the size of the UNAMIR force, along with the French and Belgian evacuation of foreign nationals, effectively gave a green light to those who sought to perpetrate atrocities. As detailed by Human Rights Watch, Annan resisted efforts to strengthen UNAMIR’s mandate and allow it to take a more active role in protecting lives.

By all accounts, Annan sought to learn from his own failures. A year later, still serving as head of U.N. peacekeeping, he authorized more aggressive military action in Bosnia. As secretary general, Annan played an important role in developing and promoting the “Responsibility to Protect” — the idea that the international community has a responsibility to intervene to prevent massive loss of human life. He also authorized an independent commission that investigated the U.N. role in Rwanda and criticized the responses of the United Nations, the United States, France and other international actors.

In his memoir, “Interventions: A Life in War and Peace,” Annan was frank about his regrets about Rwanda and discussed actions that he might have taken to prevent and mitigate the violence. At a memorial conference in New York a decade after the genocide, he said, “The international community failed Rwanda, and that must leave us always with a sense of bitter regret and abiding sorrow.”

As secretary general, Kofi Annan helped to transform and strengthen the United Nations. Learning from Rwanda, he pushed the United Nations to increase its ability to stop atrocities. In a 2012 speech, Annan expressed satisfaction in the U.N. decision to accept the responsibility to protect civilians from atrocities, saying, “The endorsement of this principle by U.N. member states in 2005 was a momentous step. It made clear that hand-wringing and appeals to conscience by the international community are not enough.”

After retiring from the United Nations, Annan continued to promote peace-building through his Kofi Annan Foundation and his work as a mediator. His failure in the face of the Rwandan genocide could be viewed as one black mark on an otherwise impressive record — yet a failure that contributed to hundreds of thousands of deaths is difficult to overlook.

Timothy Longman is associate professor of political science and international relations at Boston University and author of “Memory and Justice in Post-Genocide Rwanda” and “Christianity and Genocide in Rwanda.”