In years of overseeing international relief efforts, I have been a witness to four genocides, but Rwanda was the worst. Standing on ground saturated with the blood of 800,000 murdered people was frightening, exhausting and devastatingly painful. Everyone who survived in Rwanda did some dying in the surviving.

With Sunday marking the 20th anniversary of the beginning of the Rwandan genocide, it is worthwhile to contemplate that terrible event and what it means today as the world watches—and all too often turns away from—mass killings in the Darfur region of Sudan, South Sudan, the Central Africa Republic, Congo, Syria and elsewhere. As the world's collective memory of Rwanda dims, atrocities are still perpetrated, unchecked by the political rhetoric aimed at them.

I have always found it odd that in areas of mass death, especially violent death, the birds and insects cease to make noise. That was the first thing I noticed as I drove into Rwanda from Uganda in early May 1994. The overwhelming silence could not be ignored.

The killing had started on April 6, 1994, when a plane carrying Rwanda's president, Juvénal Habyarimana, was shot down in the capital of Kigali as it approached landing. For the next three months, Rwanda became a nightmarish killing field as ethnic Hutu supporters of the Habyarimana government hunted down ethnic Tutsis, blaming them for his death. I saw thousands of bloated bodies poke through the mist of Rizumu Falls like quivering masses of rubber dolls moving down the river into Lake Victoria.

The organization I work for, Samaritan's Purse, went to a small village named Rutare, to where 10,000 Tutsi refugees had fled. Within three weeks the population of the Rutare camp exceeded 125,000 as Tutsis scrambled north ahead of Hutu killers. We furnished medical care and clean water, and by the time we left Rutare we had assumed care for 930 parentless children.

In July, after the massacres had subsided, some of us went to Kigali. We slept on cots and ate canned food. We kept away from the windows. No one wanted to say it, but we were all afraid and frustrated. Bodies were strewn in the roads and the gutters and the ditches and the fields—piles of bodies, whole bodies and pieces of bodies.

A tall, thin, eloquent woman in the uniform of the Rwandan Patriotic Front—which had fought back against the genocidal regime and is in power today—gave permission for my team to use a house. Her name was Rose Kabuye. She had the rank of major and was later made the mayor of Kigali. I still have a blue index card on which she wrote in 1994: "Please do not vacate this man, Ken Isaacs of Samaritan's Purse, from this residence. He has permission to stay and use it." The card became the equivalent of a lease for three years.

We focused our attention on restoring the Central Hospital of Kigali, a 600-bed facility that had been looted, sacked and used for a killing ground. With our 30 staff members, we began cleaning the facility. We carried corpses stacked on canvas stretchers.

Every person I met had a story. One woman had been forced to dig a grave and decide which of her seven children she would throw in alive and bury. If she refused, all of them would be murdered. Within weeks, she had buried them all. When I met her, she was completely despondent and disfigured by machete wounds.

In 1998, President Bill Clinton visited Kigali. He apologized to the Rwandan people for failing them—for acting too slowly, doing too little. Having been there during the murderous frenzy, I will always believe that a few thousand international troops with a real mandate to fight would have prevented the genocide.

Politicians and academics talk about how much we've learned from the latest atrocity, wherever it happens to be. But there is nothing left for us to learn about mass murder. We know what it is. We continue to see it unfold in places like Sudan. United Nations resolutions have been powerless. The International Criminal Court has no power to enforce its warrants. Sudanese President Omar al-Bashir has not been arrested for his crimes against humanity. Sudan is only one example.

The time to act is not after the killing stops. The international community makes foreign-policy statements based on values, but political self-interest always rules the day when it comes to action. I've seen too much to believe it when leaders swear "never again." Putting an end to genocide is not an academic exercise. Evil can only be resisted by the resolute will to stand against it the moment genocide threatens to be unleashed, even if there is a political price to pay.

Mr. Isaacs is vice president of programs and government relations for the international relief organization Samaritan's Purse and was director of the U.S. Office of Foreign Disaster Assistance (2004-05).