Perhaps, in attempting to forecast the outbreak of a future war, we should ask another question: Why do we make war at all? War has been a characteristic of almost the whole of recorded history, and warlike violence preceded the establishment of the first states. Why human beings have developed belligerency alongside their capacity for social cooperation remains a fundamental question.

It is a puzzle with which the human ­sciences have wrestled for much of the 20th and 21st centuries. For evolutionary biologists and psychologists, warfare was a means for early man to ensure survival, protect kin and cope with ecological crisis. No human biologist now argues that violence is in our genes, but early hominins, organised in small bands of hunter-gatherers or fishers, almost certainly used violence to protect against intruders, secure resources and food, and on occasion to act as predators on neighbouring communities. The resort to violence as one of the elements in the survival kit of early man became psychologically normative, as well as biologically useful. On this reading, bell­igerence is something deeply embedded in human development.

Yet this view is challenged by the other sciences, which see warfare as a phenomenon associated with the development of settled cultures and political systems, whether tribe, proto-state or state. By 10,000 years ago, there is no doubt that something resembling warfare emerged worldwide, evidenced in the archaeological record of weapons, iconography and fortifications. 

Warfare was not like modern war, organised in mass armies and supplied by military industries, but took a variety of forms: a deadly raid, a ritual encounter, or a massacre, such as the Nataruk killings, dating to the 9th century BC: the remains of men, women (one of them pregnant) and children unearthed from this site near Kenya’s Lake Turkana show the victims were clubbed and stabbed to death.

It was evidently not necessary to have a state to engage in violence, as the tribal warfare observed in the past few hundred years has demonstrated, but war did mean the emergence of a warrior elite and a culture in which warfare was valorised and endorsed: the Spartans, the Vikings, the Aztecs. There have been very few cultures in which warfare has not played a part, usually a central part, in the life of the community. In the historic period of states, from about 5,000 years ago, there are no examples where warfare was not accepted practice.