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.
In 1945 atomic bombs were dropped on the Japanese cities of Hiroshima and Nagasaki.
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.