For thousands of years scholars have sought to understand the nature of war, and in the modern era, with two world wars and the threat of nuclear holocaust, there has been more writing on the subject than ever before. Theories of war are important because they can shape human expectations and determine behaviour. They can also influence the morality of a society, and so have a profound effect on the ways in which people act and live.

A wide range of approaches have been taken, ranging from very general and intuitive assertions about the human nature to complex analyses based on the concepts and techniques of modern psychology. Theorists in this field are aware of the powerful influence their theories can exert, and most include a normative element, as they often strive to provide insights that will help to prevent war or limit its severity.

One of the main issues is how to define war, for it is important to distinguish between actual or threatened clashes of arms and riots and rebellions, metaphorical or abstract clashes of values, and so on. Attempts to define war in different ways inevitably involve making implicit or explicit assumptions about the nature of determinism and freedom.

Theorists may posit that the root causes of war are based on human nature or the dynamics of the inter-state system. This view is often combined with a desire to understand the motivations of belligerents, which can be influenced by a variety of factors such as social development, ideological movements and changing political objectives. This approach to war has been characterised as’social darwinism’, and theorists in this area have often viewed it as a necessary stage in the development of societies.

Alternatively, some theorists hold that the root causes of war are moral, cultural and ethical. They are concerned with a variety of ethical issues, including the nature of man and his need for self-preservation and achievement, and the way in which culture and beliefs influence and sustain warlike attitudes. These views are frequently associated with a belief in the supremacy of individual rights and a desire to establish the justification for the use of force against a state that has committed unjust acts.

Others point out that warfare is inherently destructive, not only in terms of the death toll of soldiers and civilians, but also in its wider consequences. They argue that a long period of conflict can leave a country impoverished, create insecurity and lead to loss of property and the destruction of the natural environment, as well as causing severe physical and mental health problems for those involved in the fighting. These problems are even more serious when a nation has access to nuclear weapons, which can be indiscriminately used against its neighbours. These are therefore important considerations in any theory of war, and are a central theme in many debates about the morality of warfare.