The Medieval period saw the rise of military power in Europe. This was due to both the need to fend off raiders as well as the transition from feudal to centralized governments. Throughout the period, the cavalry and infantry were the two main units.

In the early Medieval period, the cavalry was the most important unit. Heavy cavalry consisted mostly of knights who were typically from nobility and could afford the required heavy armor. Because they could break through the enemy line, many battles were won due to a strong heavy cavalry. Light cavalry consisted of wealthy commoners and poor knights. They were armored lightly and fought with lances, javelins, bows, and crossbows. Some countries became know for their unique light cavalry units. Hungary was known for its mounted archers, while Spain was known for its jinetes, who fought with javelins, swords, and shields.

The infantry made up of the body of the army, often consisting of conscripted peasants. Mercenaries were also used. As governments became more centralized it became standard to have a paid standing army. Typically armed with spears or bows they were, initially, the weakest unit in the army. In an effort to more effectively combat the stronger cavalry, the infantry borrowed the phalanx formation from the ancient Greeks. The Swiss, however, broke away from this formation, instead opting for something more flexible. Their new formation was so successful that soon almost every major European leader was hiring Swiss pikemen for his army.

The English were a notable exception to jumping on the Swiss pikemen band wagon. Perhaps it is because they had developed their own effective defense again cavalry: the longbow. Though it required many years of practice, the longbow could pierce mail and dent plate armor. The crossbow had greater penetration power, but the longbow was arguably more effective, because it could be fired faster and cost less.

Eventually the pikemen and longbow ended the use of cavalry as the dominate unit in Medieval warfare. Thus, though both were essential to warfare throughout the period, how they were used changed as new tactics were formed and implemented.