This month's anniversary of the bombings of Hiroshima and Nagasaki prompts only the most sombre reflection and most fervent hope that the horror may never be repeated.
In the subsequent 60 years, those bombings have haunted the world's imagination but not so much as to curb the development and spread of infinitely more lethal weapons of mass destruction.
A related concern, discussed in technical literature well before 11 September 2001, is that nuclear weapons may sooner or later fall into the hands of terrorist groups.
The recent explosions and casualties in London are yet another reminder of how the cycle of attack and response could escalate, unpredictably, even to a point horrifically worse than Hiroshima or Nagasaki.
The world's reigning power accords itself the right to wage war at will, under a doctrine of "anticipatory self-defence" that covers any contingency it chooses. The means of destruction are to be unlimited.
US military expenditures approximate those of the rest of the world combined, while arms sales by 38 North American companies (one in Canada) account for more than 60 per cent of the world total (which has risen 25 per cent since 2002).
There have been efforts to strengthen the thin thread on which survival hangs. The most important is the nuclear Nonproliferation Treaty (NPT), which came into force in 1970. The regular five-year review conference of the NPT took place at the United Nations in May.
The NPT has been facing collapse, primarily because of the failure of the nuclear states to live up to their obligation under Article VI to pursue "good faith" efforts to eliminate nuclear weapons. The United States has led the way in refusal to abide by the Article VI obligations. Mohamed ElBaradei, head of the International Atomic Energy Agency, emphasises that "reluctance by one party to fulfil its obligations breeds reluctance in others".
President Jimmy Carter blasted the United States as "the major culprit in this erosion of the NPT. While claiming to be protecting the world from proliferation threats in Iraq, Libya, Iran and North Korea, American leaders not only have abandoned existing treaty restraints but also have asserted plans to test and develop new weapons, including anti-ballistic missiles, the earth-penetrating 'bunker buster' and perhaps some new 'small' bombs. They also have abandoned past pledges and now threaten first use of nuclear weapons against non-nuclear states".
The thread has almost snapped in the years since Hiroshima, repeatedly. The best known case was the Cuban missile crisis of October 1962, "the most dangerous moment in human history", as Arthur Schlesinger, historian and former adviser to President John F Kennedy, observed in October 2002 at a retrospective conference in Havana.
The world "came within a hair's breadth of nuclear disaster", recalls Robert McNamara, Kennedy's defence secretary, who also attended the retrospective. In the May-June issue of the magazine Foreign Policy, he accompanies this reminder with a renewed warning of "apocalypse soon".
McNamara regards "current US nuclear weapons policy as immoral, illegal, militarily unnecessary and dreadfully dangerous", creating "unacceptable risks to other nations and to our own", both the risk of "accidental or inadvertent nuclear launch", which is "unacceptably high", and of nuclear attack by terrorists. McNamara endorses the judgement of William Perry, President Bill Clinton's defence secretary, that "there is a greater than 50 per cent probability of a nuclear strike on US targets within a decade".
Similar judgements are commonly expressed by prominent strategic analysts. In his book Nuclear Terrorism, the Harvard international relations specialist Graham Allison reports the "consensus in the national security community" (of which he has been a part) that a "dirty bomb" attack is "inevitable", and an attack with a nuclear weapon highly likely, if fissionable materials - the essential ingredient - are not retrieved and secured.
Allison reviews the partial success of efforts to do so since the early 1990s, under the initiatives of Senator Sam Nunn and Senator Richard Lugar, and the setback to these programmes from the first days of the Bush administration, paralysed by what Senator Joseph Biden called "ideological idiocy".
The Washington leadership has put aside non-proliferation programmes and devoted its energies and resources to driving the country to war by extraordinary deceit, then trying to manage the catastrophe it created in Iraq.
The threat and use of violence is stimulating nuclear proliferation along with jihadi terrorism.
A high-level review of the "war on terror" two years after the invasion "focused on how to deal with the rise of a new generation of terrorists, schooled in Iraq over the past couple of years", Susan B Glasser reported in The Washington Post.
"Top government officials are increasingly turning their attention to anticipate what one called 'the bleed out' of hundreds or thousands of Iraq-trained jihadists back to their home countries throughout the Middle East and Western Europe. 'It's a new piece of a new equation,' a former senior Bush administration official said. 'If you don't know who they are in Iraq, how are you going to locate them in Istanbul or London?'"
Peter Bergen, a US terrorism specialist, says in The Boston Globe that "the President is right that Iraq is a main front in the war on terrorism, but this is a front we created".
Shortly after the London bombing, Chatham House, Britain's premier foreign affairs institution, released a study drawing the obvious conclusion - denied with outrage by the Government - that "the UK is at particular risk because it is the closest ally of the United States, has deployed armed forces in the military campaigns to topple the Taliban regime in Afghanistan and in Iraq ... [and is] a pillion passenger" of American policy, sitting behind the driver of the motorcycle.
The probability of apocalypse soon cannot be realistically estimated, but it is surely too high for any sane person to contemplate with equanimity. While speculation is pointless, reaction to the threat of another Hiroshima is definitely not.
On the contrary, it is urgent, particularly in the United States, because of Washington's primary role in accelerating the race to destruction by extending its historically unique military dominance, and in the UK, which goes along with it as its closest ally.
The author is a professor of linguistics at the Massachusetts Institute of Technology and the author, most recently, of Hegemony or Survival: America's Quest for Global DominanceReuse content