Until today, no sitting President of the United States had visited Hiroshima. In August 1945, President Truman forever etched Hiroshima into American history books, and indeed that of the world, when, with a bright flash, the city became the first victim of the atomic age.

The specter of imminent nuclear war may not feel as urgent today as it did when people were building fallout shelters, but it’s still very much there. The United States and Russia both have over 7,000 nuclear warheads. Seven other countries maintain nuclear arsenals an order of magnitude smaller: France has the third most, at 300, and North Korea has the least, at 8 (as of October 2015). North Korea has, since then, tested another nuke and revealed another warhead.

This is the context of Obama’s visit to Hiroshima, the first sitting American president to stand where the allies’ science and ingenuity unleashed a new horror onto the world.

What did Obama say on this occasion? A great many things, and the transcript is worth reading in full. The history of Hiroshima after the bomb is very much a human history, defined by compassion. Yet there was also a technological core to Obama’s remarks, and a deeper question of science and morality.

Said President Obama:

Indeed, these are the questions we wrestled with in the pages of Popular Science after news of the bombing. Our editors hoped, they wrote in the September 1945 issue, “that readers of this magazine will be stimulated to contribute to the new era of science that dawned on August 6th, 1945. By splitting the atom, man may have united the world.””

Nearly 71 years later, we’re still waiting. At the very least, the atom hasn’t annihilated the world either.