In the beginning there was no war. For millions of years, in the Old Stone Age, small bands of hominids (human-like species) roamed across the landscape seeking food by hunting, gathering, and scavenging.
Meetings were rare, and clashes of any kind rarer still.
Only later, as numbers built up, were there occasional conflicts over resources. Cave art shows hunters with bows shooting not only animals but sometimes each other.
But this was still not war. War is large-scale, sustained, organised violence between opposing groups. There is no evidence for this before the agricultural revolution took place.
Societies based on herding animals and cultivating crops started at different times in different places. In Britain, the agricultural revolution occurred around 4000 BC.
It was the greatest change in economic development until the Industrial Revolution of the 18th century.
Farming was a much more efficient way of getting food than hunting, so the population increased enormously in the New Stone Age. But there were also dangers.
Technology was primitive, productivity low and surpluses small. People lived close to the edge, prey to natural disasters like crop failure.
The threat of famine could drive neighbouring groups into conflict. Early farmers had property – such as fields, animals, storehouses, and permanent homes – to defend in hard times.
This combination of poverty and property was the root cause of the first wars. The starving might eat by seizing the grain and sheep off their neighbours.
But if you want to fight a war, you need warriors.
Not everyone can fight, and those who can become better at fighting if they train. Societies that use some of their surplus to support specialist warriors will defeat societies that do not.
Archaeologists now see the decades around 3500 BC as the time of the first wars in Britain.
Great hilltop camps were built surrounded by banks and ditches – evidence that prehistoric farmers were being organised into large political units.
Some people were buried in monumental tombs of stone and earth, which is evidence of an elite. The study of skeletons shows that many people at the time were killed by arrows or clubbing.
Archaeologists have even found evidence for a full-scale battle – a number of flint arrowheads at the great camp at Crickley Hill in Gloucestershire.
The poverty and property of early farmers had given rise to an embryonic social order based on tribes, territories and a warrior class. But it did not stop there. War quickly took on a life of its own.
By choice, most people would probably have supported wars only when they were defensive. But the warriors could free themselves from work and want by making war the norm. And they had the weapons.
Society was forced to maintain the warriors as a permanent group – and they evolved into a ruling class of exploiters.
The evidence for this from later prehistoric societies is well-known to archaeologists. The elite of Bronze Age warriors were equipped with axes and swords, those of the Iron Age with chariots and even chain-mail armour.
Wars were no longer just defensive or to stave off hunger. With the world divided into rival tribes, each headed by a warrior elite, the temptation was to attack before being attacked.
This was also a way for the ruling class to increase its wealth and power.
Technology remained primitive. Little could be done to increase yields.
Breaking in new land was hard graft. The easiest thing for the ruling class was to use aggressive war to seize the surpluses of rival tribes – to pillage grain stores, rustle sheep and cattle, turn captives into slaves, and annex farms and fields.
Military competition intensified as the most agressive ruling classes used war to accumulate surpluses and increase power at the expense of weaker rivals.
In Britain, by the end of the Iron Age, many smaller tribes had been absorbed into great confederations ruled by kings.
They built huge earth and timber fortresses, issued silver coins inscribed with their own names, and wore neckrings of twisted gold as badges of class power.
When the Roman general Julius Caesar invaded in 54 BC, the defenders of south east Britain were a military elite of 4,000 charioteers.
They represented a terrible new age in which war had become an end in itself.