Medieval warfare
Strategy and tactics: Medieval campaigns were planned with strategy in mind, such as maintaining unity in morale, planning troop movements, and mount offensives with numerical advantages. Medieval armies used strategic deception, such as misleading troop movements, to take opposing armies by surprise. They would also spread misinformation regarding army size and provisions.
One common tactic used in medieval warfare was raiding; this benefitted the attacking army by with new supplies and wealth while damaging the target's resources.

De re militari: Publius Flavius Vegetius Renatus wrote De re militari (Concerning Military Matters) possibly in the late 4th century. Described by historian Walter Goffart as "the bible of warfare throughout the Middle Ages", De re militari was widely distributed through the Latin West. While Western Europe relied on a single text for the basis of its military knowledge, the Byzantine Empire in Southeastern Europe had a succession of military writers. Though Vegetius had no military experience and De re militari was derived from the works of Cato and Frontinus, his books were the standard for military discourse in Western Europe from their production until the 16th century.
De re militari was divided into five books: who should be a soldier and the skills they needed to learn, the composition and structure of an army, field tactics, how to conduct and withstand sieges, and the role of the navy. According to Vegetius, infantry was the most important element of an army because it was cheap compared to cavalry and could be deployed on any terrain. One of the tenets he put forward was that a general should only engage in battle when he was sure of victory or had no other choice. As archaeologist Robert Liddiard explains, "Pitched battles, particularly in the eleventh and twelfth centuries, were rare."
Although his work was widely reproduced, and over 200 copies, translations, and extracts survive today, the extent to which Vegetius affected the actual practice of warfare as opposed to its concept is unclear because of his habit of stating the obvious. Historian Michael Clanchy noted "the medieval axiom that laymen are illiterate and its converse that clergy are literate", so it may be the case that few soldiers read Vegetius' work. While their Roman predecessors were well-educated and had been experienced in warfare, the European nobility of the early Medieval period were not renowned for their education, but from the 12th century, it became more common for them to read.
Some soldiers regarded the experience of warfare as more valuable than reading about it; for example, Geoffroi de Charny, a 14th century knight who wrote about warfare, recommended that his audience should learn by observing and asking advice from their superiors. Vegetius remained prominent in medieval literature on warfare, although it is uncertain to what extent his work was read by the warrior class as opposed to the clergy. In 1489, King Henry VII of England commissioned the translation of De re militari into English, "so every gentleman born to arms and all manner of men of war, captains, soldiers, victuallers and all others would know how they ought to behave in the feats of wars and battles".

Supplies and logistics: Medieval warfare largely predated the use of supply trains, which meant that armies had to acquire food supplies from the territory they were passing through. This meant that large-scale looting by soldiers was unavoidable, and was actively encouraged in the 14th century with its emphasis on chevauchée tactics, where mounted troops would burn and pillage enemy territory in order to distract and demoralize the enemy while denying them their supplies.
Through the medieval period, soldiers were responsible for supplying themselves, either through foraging, looting, or purchases. Even so, military commanders often provided their troops with food and supplies, but this would be provided instead of the soldiers' wages, or soldiers would be expected to pay for it from their wages, either at cost or even with a profit.
In 1294, the same year John II de Balliol of Scotland refused to support Edward I of England's planned invasion of France, Edward I implemented a system in Wales and Scotland where sheriffs would acquire foodstuffs, horses and carts from merchants with compulsory sales at prices fixed below typical market prices under the Crown's rights of prise and purveyance. These goods would then be transported to Royal Magazines in southern Scotland and along the Scottish border where English conscripts under his command could purchase them. This continued during the First War of Scottish Independence which began in 1296, though the system was unpopular and was ended with Edward I's death in 1307.
Starting under the rule of Edward II in 1307 and ending under the rule of Edward III in 1337, the English instead used a system where merchants would be asked to meet armies with supplies for the soldiers to purchase. This led to discontent as the merchants saw an opportunity to profiteer, forcing the troops to pay well above normal market prices for food.
As Edward III went to war with France in the Hundred Years' War (starting in 1337), the English returned to a practice of foraging and raiding to meet their logistical needs. This practice lasted throughout the war, extending through the remainder of Edward III's reign into the reign of Henry VI.

Regional examples: 

Arabs: The initial Muslim conquests began in the 7th century after the death of the Islamic prophet Muhammad, and were marked by a century of rapid Arab expansion beyond the Arabian Peninsula under the Rashidun and Umayyad Caliphates. Under the Rashidun, the Arabs conquered the Persian Empire, along with Roman Syria and Roman Egypt during the Byzantine-Arab Wars, all within just seven years from 633 to 640. Under the Umayyads, the Arabs annexed North Africa and southern Italy from the Romans and the Arab Empire soon stretched from parts of the Indian subcontinent, across Central Asia, the Middle East, North Africa, and southern Italy, to the Iberian Peninsula and the Pyrenees.
The early Arab army mainly consisted of camel-mounted infantry, alongside a few Bedouin cavalry. Constantly outnumbered by their opponent, they did, however, possess the advantage of strategic mobility, their camel-borne nature allowing them to constantly outmaneuver larger Byzantine and Sassanid armies to take prime defensive positions. The Rashidun cavalry, while lacking the number and mounted archery skill of their Roman and Persian counterparts was for the most part skillfully employed, and played a decisive role in many crucial battles such as Battle of Yarmouk. During the 7th century, Arab armies employed weapons such as swords, spears, iron mace and lances. For protection, they used shields and wore helmets and coats of mail, although the latter was extremely rare. The bow and arrow was also utilized. Also, after the naval siege of Constantinople in the 670s, they started to employ greek fire.
In contrast to the Roman and Persian army at the time both had large numbers of heavy infantry and heavy cavalry (cataphracts and clibanarii) that were better equipped, heavily protected, and were more experienced and disciplined. The Arab invasions came at a time when both ancient powers were exhausted from the protracted Byzantine–Sassanid Wars, particularly the bitterly fought Byzantine–Sassanid War of 602–628 which had brought both empires close to collapse.  Also, the typically multi-ethnic Byzantine force was always wracked by dissension and lacked a unity of command, a similar situation also being encountered among the Sassanids who had been embroiled in a bitter civil war for a decade before the coming of the Arabs. In contrast, the Ridda Wars which preceded the Arab conflicts with both the Sasanids and the Byzantines, had forged the Caliphate's army into a united and loyal fighting force.

Vikings: The Vikings were a feared force in Europe because of their savagery and speed of their attacks. Whilst seaborne raids were nothing new at the time, the Vikings refined the practice to a science through their shipbuilding, tactics and training. Unlike other raiders, the Vikings made a lasting impact on the face of Europe. During the Viking age, their expeditions, frequently combining raiding and trading, penetrated most of the old Frankish Empire, the British Isles, the Baltic region, Russia, and both Muslim and Christian Iberia. Many served as mercenaries, and the famed Varangian Guard, serving the Emperor of Constantinople, was drawn principally of Scandinavian warriors.

Viking longships were swift and easily manoeuvered; they could navigate deep seas or shallow rivers, and could carry warriors that could be rapidly deployed directly onto land due to the longships being able to land directly. The longship was the enabler of the Viking style of warfare that was fast and mobile, relying heavily on the element of surprise. The usual method was to approach a target with the element of surprise and then retire swiftly using guerrilla-style fighting. The fully armoured Viking raider would wear an iron helmet and a mail hauberk, and fight with a combination of axe, sword, shield, spear or great "Danish" two-handed axe, although the typical raider would be unarmoured, carrying only a bow and arrows, a seax, a shield and spear. European countries with a weak system of government would be unable to organize a suitable response and would naturally suffer the most to Viking raiders.