Nuclear weapons add a completely new dimension to man's powers of destruction. Published estimates of the effects of nuclear weapons range from the concept of the total destruction of humanity to the belief that a nuclear war would differ from a conventional conflict only in scale. The situation, however, is not as arbitrary as opposing generalizations such as these might suggest. There is one inescapable and basic fact: the nuclear armouries in existence already contain large megaton weapons, every one of which has a destructive power greater than that of all the conventional explosives ever used in warfare since the day gunpowder was discovered. Were such weapons ever to be used in numbers, hundreds of millions of people might be killed; and civilization as we know it, as well as organized community life, would inevitably come to an end in the countries involved in the conflict. Many of those who survived the immediate destruction, as well as others in countries outside the area of conflict, would be exposed to widely-spread radio-active contamination, would suffer from long-term effects of irradiation and would transmit to their offspring a genetic burden which would become manifest in the disabilities of later generations.
A study has been made of the likely results of a nuclear attack on a hypothetical industrial region, consisting of nine cities each with populations of over 50,000 inhabitants (some well over), and also containing 140 smaller towns of fewer than 50,000 inhabitants (about sixty of which containing elements of key industry). Assuming that a one-megaton bomb burst at ground level in each of the nine cities, the study showed that simple cumulative estimates of casualties provide a very inadequate measure of the over-all effects of the attack. Such estimates showed that 20% of the total population (or 30% of the urban population, 35% of the key-industrial population) would be killed. The houses destroyed would be 30% of total (or 40% of urban, 50% of those occupied by key-industrial population). But cities are not isolated entities; they are linked in a variety of functional ways, being dependent on each other for raw materials of different kinds, as well as for semi-finished and finished manufactured goods. Taking the interaction of effects into account, the study showed that the percentage of key industry in the whole region (that is, industry with more than local significance) which would be brought to a stop would be between 70% and 90% of the whole. The lower figure of 70% takes account of everything directly destroyed or completely disrupted inside the target cities; the higher figure of 90% includes the areas surrounding the city which would also be indirectly 'knocked out' through, for example, failures of communications or supplies of raw materials and food. The more interdependent they are, the larger is the multiplying factor one has to bear in mind when estimating the cumulative effects of the destruction of single cities.
In hypothetical studies of this kind it has also been estimated that in the absence of special protection, blast-induced deaths alone resulting from 400 high level ten-megaton bombs aimed at USA metropolitan areas, would eliminate more than half of the total American population of over 200 million people. Even if they were all in substantial fall-out shelters the same proportion would be killed if the weapons were burst at ground level.
The effects of all-out nuclear war, regardless of where it started, could not be confined to the powers engaged in that war. They themselves would have to suffer the immediate kind of destruction and the immediate and more enduring lethal fall-out whose effects have already been described. But neighbouring countries, and even countries in parts of the world remote from the actual conflict, could soon become exposed to the hazards of radioactive fall-out, in precipitation at great distances from the explosion, of matter which had moved through the atmosphere as a vast cloud. Thus, at least within the same hemisphere, an enduring radio-active hazard could exist for distant as well as close human populations, through the ingestion of foods derived from contaminated vegetation, and the external irradiation due to fall-out particles deposited on the ground. The extent and nature of the hazard would depend upon the numbers and type of bombs exploded. Given a sufficient number, no part of the world would escape exposure to biologically significant levels of radiation. To a greater or lesser degree, a legacy of genetic damage could be incurred by the world's population.
It is to be expected that no major nuclear power could attack another without provoking a nuclear counter-attack. It is even possible that an aggressor could suffer more in retaliation than the nuclear power it first attacked. In this lies the concept of deterrence by the threat of nuclear destruction. Far from an all-out nuclear exchange being a rational action which could ever be justified by any set of conceivable political gains, it may be that no country would, in the pursuit of its political objectives, deliberately risk the total destruction of its own capital city, the destruction of all its major centres of population and the resultant chaos which would leave in doubt a government's ability to remain in control of its people.