Twenty years after the Rwandan genocide he tried to stop and stayed to witness, Roméo Dallaire still has gory flashbacks.

In an interview before the U.S. Holocaust Memorial Museum dinner Wednesday at which he received the museum’s highest honor, the Elie Wiesel Award, the former commander of the U.N. peacekeeping mission in Rwanda talked about the times his post-traumatic stress disorder has made him relive the slaughter.

A dozen years ago, Dallaire said, he saw a street vendor in Sierra Leone raise a machete to chop a coconut in two and “went absolutely [berserk]” at the sight of the weapon that had been wielded with such efficiency in the massacre of more than 800,000 people in only 100 days. “It took three guys to hold me down and for 15 minutes I [was] reliving the genocide’’ in vivid detail.

At his home in Canada on Easter last year, his toddler granddaughter fell and hit her head on a coffee table. “Everybody else leapt to their feet to help her, but I couldn’t move,” he said. Because in 1994, “I saw hundreds of children killed,’’ many of them by soldiers who were only children themselves. And at the sound of the little girl’s cry, “they all came back.’’ It took him three weeks to dare to pick her up again, for fear that if she cried, he might freeze and drop her.

Only a few months ago, in December, he dozed off at the wheel one morning en route to Parliament, where he serves as a senator, after a sleepless night of flashbacks triggered by a rash of suicides by Canadian veterans of the war in Afghanistan. “I’d argued we should have been much more proactive’’ in treating and supporting the vets, but once again, he said, his warnings had been ignored.

The Nick Nolte character in “Hotel Rwanda” was based on Dallaire, although he has said the film was truer to Hollywood than to Kigali. The Holocaust museum’s Web site praises him as someone who in Rwanda “did his utmost to warn the United Nations of the potential outbreak of large-scale ethnic violence. Even when his warnings were went unheeded, he refused to give in to international apathy. He continually called for use of force, and although unable to stop the atrocities, he and his unit nevertheless managed to protect more than 30,000 lives.”

In Greek mythology, Cassandra had the gift of seeing the future but the curse of never being believed. And although real-world Cassandras are more often avoided than honored, “that’s exactly our job at the museum,’’ said its director, Sara Bloomfield, to hold up to the light the actions that “could have made history different” — in Europe in the 1930s, Rwanda in the 1990s and in the Central African Republic right now. Muslims in CAR are being targeted by Christian-dominated militias intent on answering atrocities attributed to a Muslim-dominated former government.

“We could smell what’s happening now in the CAR four years ago,’’ Dallaire said, because the recruitment of child soldiers “is a telltale sign — because they can be pushed beyond the limits [of most adults] — that a place is going to go to massive abuses.”

The world has learned a lot in the past 20 years about the precursors of genocide, and Dallaire’s work focuses on how to pull child soldiers, 40 percent of whom are girls, out from under the control of those who’ve scared and manipulated them into service. Yet in some ways, he says, we’ve gone backward since then.

“What hasn’t changed,’’ he said — leaning forward in his chair in a reception room off the main Washington Marriott Wardman Park ballroom, where the dinner was held — “is there’s no political will” to head off atrocities. “So we’re still stuck with politicians willing to manage these problems, instead of statesmen” who could prevent them.

At 67, the Dutch-born Dallaire says he’s trying to “make therapy ‘in.’ ” The author of the books “Shake Hands With the Devil” and “They Fight Like Soldiers, They Die Like Children,” he’s now working on a book about his PTSD. He says people continue to struggle with understanding it because “the developed world is very visual, very Cartesian. If you’re shot . . . you can see that, and it’s an honorable injury. But the guy in the corner who’s moody and getting drunk every night has an injury that’s perceived as not as honorable.”

Dallaire, who is in his 14th year of therapy, no longer drinks, and he takes nine pills every day, including antidepressants and “something that stops me from dreaming.’’

Isn’t that a loss, too? “Your greatest enemy is the night, so you pray for first light,’’ he said, but yes, there’s a trade-off involved in suppressing nightmares because “the psychologists need those dreams to help you move on.”

At the dinner, U.N. Ambassador Samantha Power, who wrote a Pulitzer Prize-winning book about the Rwandan genocide, said she was grateful for the “chance to give one of my heroes an award named for another of my heroes,” Wiesel, an activist and author who survived the Auschwitz, Buna and Buchenwald concentration camps.

Honoring Dallaire, Power said, was one way to remind the world of a time in which a congresswoman noted that she was getting a bunch of calls about the threats to Rwanda’s famed gorillas, “but no calls about the people.”

Though, today, too, Syrian President Bashar al-Assad “is deliberately targeting his own people, denying civilians food and practicing systematic torture, we must not give up” on efforts to feed the starving and end the bloodshed, she said.

“You, General,’’ Power said before slipping the medal around his neck, “have stood between killers and their prey” and “were doing your job when no one else was willing to do theirs.”

Escorted to the lectern by Holocaust survivor Nesse Godin, Dallaire accepted the award “in the name of the 454 African soldiers and 11 Canadians who stayed when everyone else left.”