War is the intentional and violent effort by nation-states and non-state actors to impose their will over others. This definition of war – distinct from the OED’s and Clausewitz’s straitjackets – includes international (global) and non-international (non-declared) conflict, guerrilla and terrorist actions as well as conventional battles between state actors.

This definition also recognises that the cause of war may not be a specific act or event, but rather an accumulation of factors including fear, honour, interest, survival and bellicose culture, domestic pressure and perceived injustice, ambition or opportunism and the dynamic interaction of violence, enmity, passion, chance and friction. It also recognises that, even when rationalised political objectives are pursued in a war, the outcome is often unpredictable.

A number of philosophical approaches to the causes of war have been advanced ranging from simple, intuitive assertions about human nature to sophisticated analyses utilising concepts and techniques from modern psychology. The majority of such theories, however, are concerned with a broad spectrum of innate drives which are manifested as desire to gain power, fear of loss of power, or the need to protect a group.

In his classic The Causes of War, Clausewitz asserts that the strategic calculus behind waging war involves a balance of considerations of force, opportunity and danger. Essentially, the greater the advantage in a situation, the more likely it is that the opponent will surrender. It is this ‘competitive calculus’ that underpins the military strategy of every major power.

There are those who argue that Clausewitz’s conception of war is flawed because it ignores the cultural context of warfare. In their view, a more accurate definition would incorporate the idea of war as ‘the continuous struggle for the control of territory and resources’. In this sense, they would regard fighting as an inextricable part of life – an instinctive and inexorable necessity for communities to survive and to distinguish themselves from other groups of people.

Those who argue in this vein tend to be social darwinists who embrace the concept of evolution. They believe that humans are programmed by their genes to fight and to do so for the benefit of a community, for example to protect women and children from harm or to protect heritage buildings or works of art.

There is also the concern that the inevitably destructive nature of war erodes cultural values and social cohesion. For many, this is a significant problem. The economic costs of a war can also have profound effects, both during and after it has finished. For example, shortages of food and water can lead to malnutrition and increase the risk of diseases such as cholera. They can also reduce the amount of money available for other social and economic needs. The psychological impact of war can be even more devastating. It can disrupt a person’s ability to work and maintain their daily routine which increases the risk of ill health, especially for young children. This can reduce their performance in school and even lead to dropouts.