Tomorrow at 8.15am, a minute's silence will reverberate around the world. The people of Japan will commemorate the victims of the first atomic bomb, which was dropped by an American B-29 on Hiroshima on 6 August 1945.

Half a world away, in Tehran, the new hard man of Iranian politics, President Mahmoud Ahmadinejad, will take the oath of office before the country's parliament. His presidency heralds a new era of uncertainty in Iran's fraught relations with the West over its nuclear ambitions.

In Beijing, urgent talks on curbing North Korea's nuclear weapons programme are close to collapse. And in Pakistan, efforts are still being made to roll up the world's biggest nuclear proliferation scandal. Sixty years after Hiroshima, whose single bomb killed 237,062 people, a new nuclear arms race has begun.

A crisis is deepening with Iran over its suspected nuclear weapons activities. Tehran is threatening to resume uranium conversion next week, prompting an emergency meeting of the International Atomic Energy Agency which could result in Iran being referred to the UN Security Council for possible sanctions.

At the six-party talks in Beijing, North Korea is refusing to abandon a nuclear weapons programme that could lead to another mushroom cloud over Asia.

International investigators are struggling to wrap up the lucrative black market that spread a web of proliferation across at least two continents thanks to the greed of one man: the father of Pakistan's nuclear bomb.

The scientist A Q Khan, who sold nuclear secrets to Iran, Libya, and possibly others, is now under house arrest.

Al-Qa'ida has still not been vanquished in its hideouts, while there are still fears that the terrorists could be working on the production of a "dirty" bomb that would spread radiation and panic in major cities.

In the light of the war on Iraq, which did not have nuclear weapons, second-tier nations have judged that North Korea was spared invasion because of its nuclear deterrent, and drawn their own strategic conclusions.

International attempts to renew a global pact banning the proliferation of nuclear weapons have foundered. In short, the system of safeguards aimed at preventing a repeat of the horrors of Hiroshima is in disarray.

The review of the Nuclear Non-Proliferation Treaty (NPT) by 189 states collapsed two months ago amid recriminations and accusations that the nuclear five had no intention of living up to their treaty commitments to pursue nuclear disarmament.

All signs are that the treaty intended to protect the world from nuclear peril is dead. Pyongyang has pulled out, boasting that it now has nuclear weapons, and other members such as Iran, Egypt and South Korea have been caught cheating.

But the regime had already been seriously undermined by states that remained outside the NPT and became nuclear powers: Israel, India and Pakistan. The NPT review at the UN in the spring provided a timely opportunity to tighten nuclear safeguards. Instead, the month-long conference turned into a bitter slanging match in which the US administration ignored its own record and turned up the heat on Iran and North Korea.

At the heart of the four-decades-old NPT is a "grand bargain". The five nuclear powers - US, Britain, France, Russia and China - agreed to work towards nuclear disarmament. In return, the non-nuclear states gave up any ambition to develop nuclear weapons; they agreed to open up all their facilities to inspection; and in return they were guaranteed the benefits of peaceful nuclear technology.

The big five have always been open to the charge of hypocrisy. Behind the rhetoric of disarmament, they have tried everything in their power to prevent second-tier powers from obtaining nuclear arms, while clinging on to their own nuclear arsenals despite strategic cuts. Both the US and Britain are upgrading: the Bush administration is developing nuclear "bunker busters" that can strike deep underground, while Britain has ordered a new generation of Trident missiles.

With the NPT seriously weakened, the challenge now is to keep the genie in the bottle, as regional rivalries in the Middle East and Asia risk going nuclear.

For the Bush administration, openly hostile to a UN solution, the answer has been talk or bomb: negotiate with states that already have a weapon (such as North Korea), or to take preemptive strikes against those that do not (such as Iraq). US officials say acting outside the treaty has produced results: it brought Libya back into the fold in 2003, when Colonel Muammar Gaddafi decided to scrap his weapons of mass destruction.

Yet this approach contains the risk of opening the path to nuclear blackmail, which is how North Korea has coaxed the West into compensating the hermit state in return for concessions on its nuclear programme.

As with Iran, negotiations have stalled on the North Korean insistence that it has the right to a civilian programme, if it renounces nuclear weapons.

Iran, an NPT member which insists on its treaty right to pursue nuclear power, has been infuriated by US co-operation with India, a non-member of the NPT, which blasted its way into the nuclear "club" in tit-for-tat tests with Pakistan in 1998.

In a world no longer guided by a universally accepted regime, countries are weighing the nuclear option. Arab states consider nuclear-armed Israel, and are drawing their own conclusions. Iran is hemmed in by hostile neighbours such as Israel and Pakistan. A nuclear test by North Korea could prompt Taiwan and Japan to follow down that road.

Preoccupied with Iraq, the US has decided to follow a diplomatic route in dealing with Iran. But if the Security Council fails to reach agreement on punishment for Tehran's infringement, the military option would loom again.

Israel has made no secret of its intention to halt militarily the Iranian nuclear weapons programme, as it did when it struck Iraq's Osiraq reactor in 1981, delaying but not ending Saddam Hussein's nuclear quest. But if Israel did strike, the Iranians could hit back anywhere in the region. Its nuclear programme would go underground, and the hand of the hardliners in Tehran would be reinforced. As one expert put it, an Israeli attack would be " a free pass for the mullahs".

The question now is whether nuclear deterrence works. The threat of American nuclear attack, albeit veiled, did not deter Saddam Hussein from invading Kuwait. On the other hand, North Korea's boasting of a nuclear arsenal saved it from invasion. And nuclear weapons have not - yet - been used on the battlefield.

Today, the "official" nuclear powers could annihilate the world many times over. And 40 other countries have the know-how to join their club. Sixty years after Hiroshima, who can say with confidence: "Never again"?

Never again?

60 years since the first use of a nuclear weapon in war. 160,000 people died when the bomb was dropped at 8.15am on Hiroshima, with another 77,062 dying later.

$27bn is spent each year by the US on nuclear weapons and related programmes

11, 000 active, deliverable nuclear weapons in the world. The US has 6,390, Russia 3,242 and Britain 200

15,654 sq miles, total land area used by US nuclear weapons bases and facilities

4 other states known or thought to have nuclear weapons: India, Israel, Pakistan, North Korea

5 acknowledged nuclear states: China, France, Russia, United Kingdom, United States

1 number of islands vaporised by nuclear testing: Elugelab, Micronesia, 1952

16 in length of 'Davy Crockett', the smallest nuclear weapon ever produced

40 states with technical ability to make nuclear weapons, including Egypt and South Korea

30,000 Kazakh conscripts served at Semipalatinsk, the Soviet test site. There were 456 tests conducted between 1945 and 1991 at the site

100 maximum number of those Kazakh conscripts still alive today

200 estimated number of nuclear weapons possessed by Israel

0 estimated number of nuclear weapons possessed by all the Arab states

100,000 people were members of the Campaign for Nuclear Disarmament in 1984

150 estimated number of nuclear weapons possessed by India

75 estimated number of nuclear weapons possessed by Pakistan

40, 000 people are currently members of CND

900 years is the time it will take for radioactive elements in Pripyat, near Chernobyl, to decay to safe levels following the disaster 19 years ago