The order to "duck and cover" sent nervous school kids cowering under their desks in the Cold War years, as all-too-real rehearsals for a nuclear blast were held across the United States.

Today, with the era of backyard bomb shelters long buried, fears of Iran's nuclear ambitions, Pakistan's shadowy stockpile, and North Korea's atomic outbursts have taken centre stage.

Yet until now, efforts to talk down a huge global nuclear arsenal of some 24,000 warheads have stalled, and nuclear and non-nuclear states are deadlocked on treaties to ban the spread of deadly weapons, clamp down on production of bomb material, and stop development of new nuclear arms.

This week, as diplomats gathered at the United Nations for talks leading up to a 2010 review of the Nuclear Non-Proliferation Treaty – the cornerstone of global nuclear disarmament – hope crept back into a gathering whose prospects for progress were bleak for most of the past decade.

After years of squabbling over prickly issues such as Iran's nuclear program and Israel's supposed atomic arsenal, they were able to agree on an agenda for the landmark conference, including disarmament, non-proliferation and the peaceful use of nuclear energy. Many credited U.S. President Barack Obama for remoulding the confrontational style of the Bush administration.

Since his election in November, Obama has energized the disarmament debate, even going so far as to visualize a world without nuclear weapons.

Obama promised to push the U.S. Congress to ratify a ban on nuclear testing and halt global production of fissile material, as well as reboot talks to shrink America's and Russia's nuclear arsenals.

But, he has warned, the world is facing new and dangerous challenges from nuclear proliferation.

"In a strange turn of history the threat of global nuclear war has gone down, but the risk of a nuclear attack has gone up," he said last month. "Black market trade in nuclear secrets and nuclear materials abound. The technology to build a bomb has spread."

The 41-year-old non-proliferation treaty has failed to control the spread of "loose nukes" and limit the number of states with nuclear weapons. But it has had some resounding successes.

It sprang from a bargain between nuclear weapons states that pledged to dismantle their arsenals and non-nuclear countries that agreed not to develop their own deadly weapons in return.

Some former Soviet nuclear states surrendered their nuclear arms, and others gave up pursuing the path to an atomic bomb. But while 189 countries have signed on to the treaty since 1968, three key states – India, Pakistan and Israel – have not, developing their own nuclear arms over international protest. North Korea defied the treaty to test a nuclear weapon, and has threatened to expand its weapons program. Iran is also suspected of working on a nuclear bomb.

Obama has pledged to speed up arms control agreements with Moscow, seeking a new treaty that would chop about one-third of the two countries' stockpiles, and open the door to even deeper cuts. But France, Britain and China have not followed suit.

Some experts warn that failure to dismantle the nuclear arsenals may have bred a dangerous cynicism in states that now have nuclear ambitions.

"Obama sets a very high standard," says non-proliferation expert William Potter of the Monterey Institute of International Studies. "But (Washington) could go in with great political will, and more flexible positions, and be surprised when other countries won't budge and we're stuck."

The most grave challenge to the Nuclear Non-Proliferation Treaty, Potter says, is its failure to deal with the possibility of nuclear terrorism.

But, he adds, countries like Canada, which belong to a group of nuclear technology suppliers, have also violated their treaty responsibilities by endorsing a deal to lift a ban on nuclear trade with India.

For some, the gaps and contradictions mean it's time to scrap the document and start fresh with one tailor-made for the 21st century.

"There are too many unsolvable problems," argues Ramesh Thakur, director of the Balsillie School of International Affairs in Waterloo. "For example, you can't accept India, Pakistan and Israel in the treaty because they would have had to conduct their nuclear tests before Jan. 1, 1967."

That keeps those countries permanently beyond the nuclear fringe, and invites more to join them in developing deadly weapons, he said. "The question is, do we think nuclear weapons are terrible, in which case everyone should disarm? Or do we think they are bad only in some countries' hands?"