There are several IHL principles we could address. We shall concentrate on the principle of discrimination. This says that it is unlawful to direct an attack against the civilian population or civilian objects as such; and only military objectives are legitimate targets of attack. Furthermore even a military target must not be attacked if civilian death or injury is excessive in relation to the concrete and direct military advantage anticipated from the attack.
The heat and explosive effects of nuclear weapons are immensely destructive. However this level of devastation can be achieved by a sufficient concentration of conventional weapons. More died in the fire bombing of Tokyo than in Hiroshima. So our starting point is that what makes nuclear weapons unique is their radioactive fallout. This can cause widespread death and suffering over a wide area and affect future generations.
The best evidence of Government thinking on this issue is in the 1995 UK written and oral pleadings before the ICJ. The UK argued that if nuclear weapons were used the intention would be to destroy military targets through their heat and blast. Radiation, said the UK, is only a side effect. There would therefore be no actual intention to "poison" the enemy through radiation (UK oral pleading 1995 para 3.60).
However, nuclear weapons are "explosive devices whose energy results from the fusion or fission of the atom." (ICJ 1996 Advisory Opinion, para 35). Radiation is therefore of the essence. The UK might believe that consequences which are inevitable and necessary, but unintended, are not relevant to the legal argument. If this is so, it has to be argued, not merely asserted.
The argument about the relationship of intention to the use of of nuclear weapons may owe something to the thinking of Sir Michael Quinlan, former Permanent Secretary at the Ministry of Defence. A recent expression of this can be found in his last book, Thinking About Nuclear Weapons. His conclusions are summarised and disputed by Brian Wicker of Pax Christi in a paper which argues that a correct analysis of action and intention makes proportionate and discriminate use of nuclear weapons impossible.
The UK pleadings emphasised the accuracy of small nuclear weapons detonated in isolated areas. These may not violate the IHL principle of discrimination. This, it is argued, would depend on the circumstances prevalent at the time.
We accept that targeting may well be accurate. However, the likely effects of a weapon must also be taken into account when assessing discrimination. No one could reliably forecast the complex atmospheric conditions and the direction of the wind at any given moment. The effects would be so unpredictable that accurate targeting would be irrelevant. No nuclear launch could be made with any assurance that its effects would fall within the bounds of legality.
Weapons like the 100 kiloton Trident warhead are designed to detonate as air bursts to cause the maximum damage. Smaller 1-5 kiloton weapons would be exploded on the ground in order to destroy precise targets. They would throw up enormous quantities of radioactive dust which would be sucked into the stratosphere and come down anywhere - even thousands of miles away. This would irradiate unpredictable numbers of people then and well into the future.
Both the UK and the US have consistently asserted that those arguing for illegality claim that all nuclear weapons have certain "inherent" characteristics which inevitably make their threat or use incompatible with international humanitarian law. "Many of the submissions made to the Court have have displayed a similar tendency to assume that, as a matter of course, any use of a nuclear weapon will inevitably violate the principles of the law war designed to protect the civilian population." (UK oral pleading page 40).
