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In the medieval period, people went to war for many reasons. Monarchs and nobles fought for political control, by taking land and demanding loyalty. They fought in order to expand their empires and territories, or to defend them against invaders. At every opportunity, medieval people tried to make economic gains through new resources or trading opportunities, and also spread and defend their religion. This generated conflict.

In Europe, the religious influence on war was obvious, as for two centuries after 1095, Christian Europeans fought Arabs in the Holy Land of the Middle East. These Crusaders fought against Muslims, initially to defend Jerusalem. In Wales, significant campaigns were fought against the English by figures such as Owain Glyndŵr.

The constant pressure of fighting led to a greater use of infantry, especially after the Black Death seriously reduced the population. The old idea of recruiting men through the feudal system was changing, and when Edward I began his conquest of Wales in 1277, many of his men were mercenaries.

New technology led to changes in attack and defence. The development of the crossbow led to plate armour, and stronger castles led to better siege machinery and tactics, such as undermining. By the 15th century, warfare had become larger-scale, more technological and more organised.