War is a state of conflict between states, or among other social organisations with some degree of organisation. The term may also be applied to the many conflicts involving non-state actors, such as guerrillas and terrorist groups. The number of ongoing wars has fluctuated throughout history and today, there are a large number of such conflicts involving both violent and non-violent means to achieve goals that often differ from the interests of those involved.

One definition is that war is any violent or coercive interaction between human beings in the pursuit of objectives that conflict with those of other humans and that can only be resolved through the use of force. This approach reflects a number of thinkers from a wide range of traditions, such as Karl Marx (war is the politics of economics), Thucydides (fear, interest and honour), Hobbes (competition, diffidence and glory) and Clausewitz (war as the continuation of politics by other means).

Another definition is that war is any act of violence between nations or between societies. This is more inclusive and allows for a broad range of interactions and conflict, from sporadic riots to prolonged territorial disputes. It also allows for the exploitation of resources and the arousal and mobilisation of manpower for both military and civilian purposes. The term war has also been used to refer to the clash of opposing systems of thought, such as religious doctrines or trading companies. The Oxford Dictionary of English expands on this by describing war as any active hostility or struggle between living beings.

Both definitions have advantages and disadvantages. One is that it is easier to study the phenomenon of war using this approach. It is, however, more difficult to identify continuities in the conduct of war. This is because the experience of combatants is always new and unique. Military personnel, for example, will encounter new situations in every deployment and will inevitably have to adapt their thinking to them.

The other disadvantage is that this approach can lead to an overemphasis on man’s rationality as the cause of war, rather than taking into account deeper drivers. This is particularly the case when it comes to a biological explanation of war. Such an approach arguably ignores the many ways in which inherited pugnacity and deep cultural structures may perpetuate warfare.

Moreover, such an emphasis may ignore the profound costs of war for society and individual. For example, in the wake of World War II, American democracy incurred an enormous cost in terms of rebuilding Western Europe. This was a consequence of the Marshall Plan, which called upon America to bear a substantial share of the burden of the rebuilding of Western Europe. Ultimately, this cost was paid by the American people and their children, who will continue to live with its legacy for generations. This is just one example of the many costs of war that are not easily quantified. Yet, they must not be ignored.