Seventy years ago today, at 8:15 in the morning, a bomb named "Little Boy" exploded 2,000 feet over the heads of two little girls, Kiyoko Tsutsui and her best friend Kazuko Aohara, while they were changing their shoes.

The girls were behind a concrete wall at Hiroshima's Honkawa Elementary School, where they were sixth-graders. When Little Boy transformed from a 10-foot-long bomb into a 1,200-foot-wide fireball hotter than the sun, the world inside the girls' school building went bright white. The girls survived, and were evacuated that night.

Kazuko's father found her and took her away, but Kiyoko's family never came. Years later, she learned that Little Boy had knocked a pillar onto her 4-year-old brother, crushing him in front of their mother. Her parents died within the week from radiation sickness. So did Kazuko, days after her father picked her up.

Kiyoko was the only survivor among her 400 classmates.

Kiyoko was sent to live with distant family, where — like many hibakusha, or atom bomb survivors — she was stigmatized and ill treated. As Kiyoko grew into a young woman, Little Boy grew with her: as cancer in her pancreas, and then thyroid, and then colon; and then meningioma and unremitting pain. Despite her illnesses, and the prejudice against hibakusha, she married and became pregnant. She hoped to give birth and start a family. But Little Boy would not allow it. Miscarriage followed miscarriage.

Today, 70 years after the bomb, in her ninth decade of life, Kiyoko lies in a hospital bed in Japan. She needs a dose of morphine every four hours, around the clock, every day, to endure the pain of the cancer planted in her on August 6, 1945. In the hallways of her hospital, her voice can be heard crying out, "Please kill me."

The hibakusha know nuclear weapons better than all the world's technical experts combined; they know what it is to suffer them.

As preachers, we know that the Bible's Psalms tell us that "the days of our years are threescore and ten." By that counting, then, the bomb has grown old, too. As we mark the 70th anniversary of the bombing of Hiroshima, however, it appears that the lessons of 70 atomic years have gone unlearned. Policymakers, including President Obama, are still deferring decisive action on disarmament to the future generations whose very existence these weapons threaten.

To many hibakusha, it seemed an answer to prayer when the newly inaugurated President Obama described his nuclear policy vision in Prague on Palm Sunday in 2009: "As the only nuclear power to have used a nuclear weapon, the United States has a moral responsibility to act. …So today, I state clearly and with conviction America's commitment to seek the peace and security of a world without nuclear weapons."

This line — and the ambitious disarmament and nonproliferation agenda that the speech laid out — are arguably what won Obama that year's Nobel Peace Prize. Six years later, however, his bold declaration of nuclear abolition seems less prophetic than the throat-clearing caveat that immediately followed: "I'm not naïve. This goal will not be reached quickly — perhaps not in my lifetime."

A lifetime is hard and short, according to the Psalmist. We get 70 years, "and if by reason of strength they be fourscore years, yet is their strength labor and sorrow; for it is soon cut off, and we fly away." A lifetime has passed since 1945. The years have been labor and sorrow, even for strong survivors like Kiyoko. Some August soon, the all-hibakusha choir that sings every year at the Nagasaki anniversary ceremony will dwindle to a solo voice, and then go silent forever.

The power of nuclear weapons is such that we instinctively scrabble at theological language to describe them. It has been this way since the first nuclear test, code-named Trinity, where bomb creator J. Robert Oppenheimer famously misquoted the Hindu scriptures, "I am become death, destroyer of worlds." The terrible consequence of such linguistic grasping, however, is that we come to treat the bomb as an implacable god or force of nature over which we have no control, and then bow before the work of our own hands in the manner of all idolatry.

This is why Obama's caveat was so devastating and, in hindsight, unsurprising. When he took the podium in Prague there was no one on Earth who had more power over the fate of nuclear weapons. But in one rhetorical stroke — "perhaps not in my lifetime" — he granted nuclear weapons functional immortality.

In hindsight, the president's nuclear policy since Prague is no surprise. The administration has pursued a series of strategic moves, foretold in the Prague speech, which reduced Cold War bloat, secured nuclear materials against terrorism, and enhanced nonproliferation — including John Kerry's hard-won deal with Iran. These are all worthy steps. In fairness, too, a hostile Congress on one side and an aggressive Russia on the other have curtailed further incremental progress.

But at the same time, in arenas that command less media attention, in ways that defy any justification, this administration has opposed efforts that would change the nuclear game or threaten the indefinite maintenance of the nuclear status quo.

Domestically, Obama fired the most anti-nuclear member of his cabinet, Chuck Hagel, while utterly failing to bring the sprawling executive branch nuclear bureaucracy into line with the ostensible vision of Prague.

Abroad, the White House has been quietly been working to undermine a growing movement, supported by over 110 governments and hundreds of civil society organizations, including the Red Cross, to "fill the legal gap for the prohibition and elimination of nuclear weapons." That is: a legal ban or convention, such as already exists for chemical and biological weapons, cluster munitions, landmines, and other armaments that specialize in killing children.

But worse even than this diplomatic offensive is the administration's own budget proposal, locking in $1 trillion for nuclear modernization over the next three decades. By the time its decisions can be reconsidered, Obama will be in his 80s. With actions like these, "perhaps not in my lifetime" hardly needs the qualifier.

Perhaps the president ought to return that Nobel medal.

The moral courage regarding nuclear weapons that President Obama promised — but has failed to deliver — is not just a transcendent ideal, but also a pragmatic imperative. These are weapons whose singular quality is their exponential multiplication of the human capacity for destruction. If we, their makers, do not cut off their years, they will one day fly away. They will be used.

This is not prophecy or special revelation. This is just the smart money when considering human fallibility alongside weapons that require perpetual perfection. And when they are used — by accident, in an act of terrorism, in regional or global war — perhaps we will no longer have to suffer this world's terrible surfeit of giggling sixth-grade girls.

William Swing is a retired bishop of the Episcopal Church and president and founder of the United Religions Initiative. Tyler Wigg-Stevenson is chair of the Global Task Force on Nuclear Weapons for the World Evangelical Alliance and the author of The World is Not Ours to Save.