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The atomic bombing of Hiroshima, Japan in 1945 was the beginning of a new era of warfare. But speaking in Hiroshima, president Obama argued that it also signaled the beginning of a new moral era, one that challenges us to change the way we think about humanity and technology. And he warned of dire consequences if we don’t.

In a ringing speech at the Hiroshima Peace Memorial, Obama said the nuclear attacks that ended World War II have a clear, simple legacy: We now have the ability to destroy ourselves. As the first sitting US president to visit Hiroshima, Obama didn’t offer an apology for the bombing. But he said the best way to honor its victims is to advance “the radical and necessary notion that we are part of a single human family.”