The radius of total destruction was about 1.6km, which resulted in fires across 11sq km. Somewhere between 70,000 and 80,000 citizens of Hiroshima were killed immediately or very soon after.
A further 70,000 citizens were injured, and had to contend with internal bleeding and cancers caused by the radiation. Three days later, after it was clear that Japan would still not surrender, a second atomic bomb, the Fat Man, was dropped on to the city of Nagasaki.
Although Japan surrendered on August 15, the nuclear genie was out of the bottle, and despite all rhetoric and feeble attempts at control, it replicated quickly.
At the high-tide point of the Cold War in the mid-1980s the total nuclear stockpile was about 50,000 warheads. Despite a number of close calls, accidents and mistakes, Armageddon was avoided.
The primary reason we are still here is because the weapons were largely restricted to two sides and both operated well-controlled and centralised systems. Critically, both acted rationally in their mutual self-restraint. Both knew that if they unleashed their nuclear weapons, mutually assured destruction would follow in reply.
Since the Cold War, successive treaties have reduced the number of nuclear weapons on the planet, down to about 10,000 today.
Of these, approximately 4000 are operationally available, and some 1800 are on high alert, able to be dispatched within 15 minutes of getting word to launch.
More than 93 per cent of the total is shared between Russia and the US. Britain, France and China are also established members of the club, while the new members such as India, Pakistan, North Korea and Israel each possess between dozens and hundreds of nuclear bombs.
Each of these weapons has evolved since Hiroshima. What once took a large military industrial complex and billions of dollars to create, can now be assembled with relative ease, and delivered in a size no larger than a standard rubbish bin.
Moreover, a contemporary maximum nuclear warhead has a yield capacity of 50,000 kilotonnes - nearly 3000 times more powerful than in 1945.
Between the blast, radiation and nuclear winter that would follow, humanity could probably destroy the Earth with only 100 of these weapons.
Despite all the recent provocations between the US, Russia and China, the greatest risk to humanity is nuclear terrorism. Terrorists, and religiously inspired ones most of all, are often not seeking rewards in the temporal realm, as suicide bombers often show.
This means they are not held in check by the rational fear of mutually assured destruction.
We know that many terrorist groups want nuclear bombs. To make one requires about 20kg of highly enriched uranium or 8kg of plutonium.
Although only a snapshot of a much larger black market, we also know that since 1995, there have been 424 recorded incidents involving illegal attempts to procure or sell nuclear materials.
Sixteen of these involved highly enriched uranium or plutonium.
The largest bust so far has been for 3kg of highly enriched uranium.
The sources have been either rogue nations, such as Pakistan and North Korea, or poorly guarded military or civilian facilities. The global stockpile of highly enriched uranium is about 1380 tonnes.
If illegally obtained, built and primed, the next nuclear bomb used in anger will probably not be delivered by a bomber flying overhead at 8:15 in the morning between countries in a state of war, but will probably flow down the same illegal routes that are used for the smuggling of drugs, people or wildlife, while the victims are in a state of peace and the enemy is largely invisible.
The risk of such an attack is believed to be between 30 and 50 per cent within the next 10 years.
• Alexander Gillespie is pro vice-chancellor of research and professor of law at the University of Waikato. He is the author of a three-volume set, A History of the Laws of War (Oxford).
