The waging of war has never been a pure free-for-all. Every culture has had a sense of limits: when war could be legitimately declared and how it would be legitimately waged. For ancient civilisations, war was a means of preserving the cosmic order. The ancient Egyptians believed their wars had to be sanctioned by the gods. Under the Zhou dynasty, Chinese armies would wage war only after oracles were consulted. Similar patterns are observable from the ancient Hindus to the North American Indian tribes.
The Second Lateran Council in 1139 banned the crossbow and ballista, the weapons of mass destruction of their day, because these armour-piercing instruments were considered too violent. By the 1300s a body of customary law had come into being to regulate the behaviour of knights, the jus militare (law of the knights), or what we today call ‘chivalry’.