A conflict between nations, or between two factions within a nation, that involves the use of military force to achieve political ends. Often war involves several belligerents, and can last for years or even decades. It may involve civilians and non-combatants, including prisoners of war, medical and religious personnel, and families of soldiers killed or wounded in a conflict. War may also involve significant economic costs, such as destruction of property and loss of production.

A large number of theories have been developed about the causes of war. One of the most important has been that war is an inevitable consequence of the inherent competition and rivalry between human groups. This theory is based on man’s innate drives, as espoused by ethologists who draw analogies with animal behaviour, and by psychologists and psychoanalysts.

Other theories have focused on the influence of culture. Some believe that the rise of civilised societies has reduced the tendency towards war, but only because of an increased awareness of the moral and ethical consequences of violence, and the more effective use of intelligence to acquire critical resources and to neutralise threats.

Yet despite this change in the moral sensibilities of human societies, it seems unlikely that the nature of war will change completely. There have always been chance events and friction that can cause a change in the direction of a war, and such occurrences can arise from human error (such as misreading intelligence or misunderstanding adversaries), technological failure, unexpected weather conditions, or even a change in the political climate.

In the past, many of these changes in war’s nature were heralded by the introduction of new weapons and techniques. The invention of gunpowder and the subsequent constant improvements in firearms are among the most widely cited examples of this. But the nature of war remains a dynamic process, in which the development of different types of weapons, the evolution of rivals’ interests and motives, and the dynamic interaction between adversaries creates unpredictable ebbs and flows that can have dramatic effects.

Although there are now more belligerents than ever before, and a larger range of methods of warfare than at any time in history, war continues to be an inescapable part of the human condition. Its enduring features are violence, enmity and passion, fear, honour, interest, survival, uncertainty and risk, bellicose culture, rationalised political objectives, and dynamic interaction between adversaries. This reflects the fundamental fact that war is still about power. And that power, ultimately, still lies with the State.