"We must tell our children about a crime unique in human history," Obama said at the U.S. Holocaust Memorial Museum. "The one and only Holocaust -- 6 million innocent people -- men, women, children, babies -- sent to their deaths just for being different, just for being Jewish."
He said future generations also must be told about the Poles, Catholics and Gypsies and gay people systematically killed by Nazi Germany forces during the World War II era also must not be forgotten.
"Let us tell our children not only how they died, but also how they lived -- as fathers and mothers, and sons and daughters, and brothers and sisters who loved and hoped and dreamed, just like us," Obama said.
During his speech, Obama announced he signed an executive order that targets technology and social media used for human-rights abuses in Iran and Syria "because national sovereignty is never a license to slaughter your people,"
He also announced that Jan Karski, whom Obama described as "young Polish Catholic, who witnessed Jews being put on cattle cars, who saw the killings, and who told the truth, all the way to President Roosevelt himself," would be honored posthumously with the Presidential Medal of Freedom, America's highest civilian honor. Karski died in 2000.
Adults must teach their children "because remembrance without resolve is a hollow gesture," Obama said. "Awareness without action changes nothing. In this sense, 'never again' is a challenge to us all -- to pause and to look within."
"Never again" is a challenge to "reject hate in all of its forms" because it has no place in a civilized world, the president said.
"Never again" is a challenge to defend fundamental rights of freedom for people and nations to exist in peace and security, as well as a challenge to societies and nations, he said.
"It's a bitter truth -- too often, the world has failed to prevent the killing of innocents on a massive scale," Obama said. "And we are haunted by the atrocities that we did not stop and the lives we did not save."
Even with all that's being done to the new sanctions and the work of organizations to raise awareness and prevent human-rights abuses and atrocities, "we know that our work will never be done," Obama said.
But, Obama said, survivors such as Elie Wiesel, the Nobel Peace Prize laureate who introduced the president, "show us the way.
"If you can continue to strive and speak, then we can speak and strive for a future where there's a place for dignity for every human being," the president said. "That has been the cause of your lives. It must be the work of our nation and of all nations."