The world was shocked when an estimated 800,000 Rwandans were massacred in about 100 days, starting April 7, 1994.
Members of the ethnic Hutu majority killed about 70 percent of the minority Tutsis living in Rwanda, and many moderate Hutus.
U.S. President Bill Clinton was criticized at home and abroad for doing nothing to stop the slaughter. Four years later, he visited Rwanda and apologized for his inaction.
"We did not act quickly enough after the killing began. All over the world, there were people like me sitting in offices day after day after day, who did not fully appreciate the depth and the speed with which you were being engulfed by this unimaginable terror," he said.
Despite the vow of "never again," the catastrophe in Rwanda did not bring the end of widespread violence against civilians.
Genocide in Sudan's Darfur region and sectarian violence in the Central African Republic are among the acts of mass brutality since then that have left millions dead or homeless.
In the Syrian civil war, at least 150,000 people are believed to have been killed, and President Barack Obama has been accused of inaction.
Amnesty International's Adotei Akwei said opposition from Russia in the United Nations Security Council is blocking White House efforts to stop the violence in Syria.
"The international community has to end the crisis in Syria somehow, and the administration has been consistent in trying to do that, but it's not been able to actually achieve the results that we all want to see," Akwei said.
Veteran U.S. diplomat Johnnie Carson, now a senior adviser at the U.S. Institute for Peace, said the Rwanda genocide taught the world it has to move quickly.
"Act swiftly, and to muster the resources of the international community to condemn and engage as quickly as possible when we see an atrocity about to occur," stated Carson
Ambassador Carson said the lessons of Rwanda and other massacres led Obama to establish an Atrocities Prevention Board, "...which is an inter-agency group that gets together once a month, under the direction of the National Security Council, to review situations around the world that may be likely to turn into atrocities or mass violence."
Akwei said the board is a "major step forward," but efforts to prevent atrocities must start sooner.
"It would be better and probably more economical to try to challenge intolerance and hate propaganda earlier on, before all of the actors are in place when things are actually beginning to threaten people," he said.
Akwei said the best way to prevent genocide is to show that the world is watching and will not tolerate it.