War is violence between states, armed factions within a state, non-declared wars by guerilla groups or other such organizations outside the control of the state, and armed conflict with civilians. It is a terrible and destructive event for the soldiers engaged in it, for the people of the country involved, and for humanity as a whole. The human cost of war – both the direct deaths caused by fighting and the indirect deaths resulting from the disruption of food production, water supply, sanitation, healthcare and the spread of diseases such as cholera – is immense and is likely to be the greatest in human history.

Contemporary theories of war divide into two broad schools: those that attribute its origin to man’s innate psychological and biological drives (e.g. ethologists drawing on analogies with animal behaviour), and those that attribute it to particular social relations and institutions. The latter school includes optimists and pessimists concerning its preventability.

A significant aspect of the nature of war is that, in addition to requiring military force to engage in it, there are strong social incentives not to fight. Nations are aware of the enormous costs of fighting and do everything they can to avoid open war. This fact makes the results of a war all the more tragic.

Although war has evolved enormously over the centuries, there are a number of fundamental continuities. The main ones are: the need to contend, bellicosity, chance and friction, a cognitive element that is based on the ability to assess relative power, and the capacity to use a range of resources to create a strategic advantage.

It is also important to remember that war is not simply a military conflict; it is a political, economic and cultural struggle. The latter inevitably involves a struggle to define and enforce social norms, particularly those relating to sexual and reproductive rights. This explains why there are always controversies over the right to abortion, contraception and homosexual marriage – even when they are not directly related to the cause of a war.

There is also a constant reminder that a war, regardless of its justification or its prosecution, inevitably diverts resources away from the more pressing needs of a population. This is especially true when it is fought between countries that have the means to produce nuclear weapons. It is, in effect, a theft from those who are hungry, malnourished and sick. In this sense it is perhaps a crime against humanity as a whole. This is a powerful insight because it means that, whatever the causes of a war, every gun that is fired and bomb that is dropped is a theft from those who are dying from hunger, disease and starvation. That is why there are so many calls for a ban on war.