"As the only nuclear power to have used a nuclear weapon, the United States has a moral responsibility to act." These words, spoken by President Barack Obama in Prague in 2009, helped guide his efforts to reduce the importance of the role nuclear weapons play in global affairs.

But they also made possible his historic visit to Hiroshima. For all of the progress that President Obama made in addressing the danger posed by nuclear weapons, it was his visit to Hiroshima that, in many ways, was the most courageous step, because it showed again that people can make history by doing things others consider too hard.

For decades, American political analysts thought it was too controversial for Americans officials to address the responsibility our country faced for the use of nuclear weapons in Japan at the end of World War II. While former presidents avoided the issue, or would mark the anniversaries of Aug. 6 or 9 with dry, short statements by low-level officials, President Obama understood that we must learn from our history and use it for a higher purpose.

While debates about nuclear weapons will continue for many years, the world has long been unified in wanting to prevent these weapons from ever be used again. And so policies under President Obama owned the moral responsibility for the invention and use of nuclear weapons and used it to drive policies that reduce the risks of nuclear use.

Sadly, those risks remain far too high. North Korea advances its nuclear capabilities, and Russia, India and Pakistan continue to increase both the size of their arsenals and their reliance on these fearsome weapons. America, under its new president, still has a moral responsibility to act and reduce the risks nuclear weapons pose to our own people and to all people.

The policies of the new administration in Washington are not yet clear. Regardless of their policies toward nuclear-armed states or the global regime that has helped prevent the spread and further use of nuclear weapons, America will always have a special role and moral responsibility to ensure no city, no people, no generation ever have to face the horrible consequences of nuclear weapons use.

When President Obama visited Hiroshima, he said that "we have a shared responsibility to look directly into the eye of history and ask what we must do differently to curb such suffering again." Such acts are hard, but American and Japan together must continue to do just that, to accept our shared responsibility and ensure that nuclear weapons are never again used, even as we seek to protect ourselves against real dangers.

There are no easy, simple answers in this effort, but as President Obama made clear, we will accept our special responsibility to act and I believe we will continue to do this under this and all future presidents because that is what history demands of us.

 opinion

(Jon Wolfsthal is associate of Belfer Center of Harvard University J.F. Kennedy School. From 2014 to 2017, he served as special assistant to former U.S. president Barack Obama as senior director for arms control and nonproliferation at the National Security Council.)