Cavalry had the advantage of improved mobility, and a man fighting from horseback also had the advantages of greater height, speed, and inertial mass over an opponent on foot. Another element of horse mounted warfare is the psychological impact a mounted soldier can inflict on an opponent.
The speed, mobility and shock value of the cavalry was greatly appreciated and exploited in armed forces in the Ancient and Middle Ages; some forces were mostly cavalry, particularly in nomadic societies of Asia, notably the Mongol armies. In Europe cavalry became increasingly armoured (heavy), and eventually became known for the mounted knights. During the 17th century cavalry in Europe lost most of its armor, ineffective against the muskets and cannon which were coming into use, and by the mid-19th century armor had mainly fallen into disuse, although some regiments retained a small thickened cuirass that offered protection against lances and sabres and some protection against shot.
In the period between the World Wars, many cavalry units were converted into motorized infantry and mechanized infantry units, or reformed as tank troops. However, some cavalry still served during World War II, notably in the Red Army, the Mongolian People's Army, the Royal Italian Army, the Romanian Army, the Polish Land Forces, and light reconnaissance units within the Waffen SS. Most cavalry units that are horse-mounted in modern armies serve in purely ceremonial roles, or as mounted infantry in difficult terrain such as mountains or heavily forested areas. Modern usage of the term generally refers to specialist units equipped with tanks ("armored cavalry") or aircraft ("air cavalry").
In many modern armies, the term cavalry is still often used to refer to units that are a combat arm of the armed forces which in the past filled the traditional horse-borne land combat light cavalry roles. These include scouting, skirmishing with enemy reconnaissance elements to deny them knowledge of own disposition of troops, forward security, offensive reconnaissance by combat, defensive screening of friendly forces during retrograde movement, retreat, restoration of command and control, deception, battle handover and passage of lines, relief in place, linkup, breakout operations, and raiding. The shock role, traditionally filled by heavy cavalry, is generally filled by units with the "armored" designation.
Before the Iron Age, the role of cavalry on the battlefield was largely performed by light chariots. The chariot originated with the Sintashta-Petrovka culture in Central Asia and spread by nomadic or semi-nomadic Indo-Iranians. The chariot was quickly adopted by settled peoples both as a military technology and an object of ceremonial status, especially by the pharaohs of the New Kingdom of Egypt as well as the Assyrian army and Babylonian royalty.
The power of mobility given by mounted units was recognized early on, but was offset by the difficulty of raising large forces and by the inability of horses (then mostly small) to carry heavy armor. Cavalry techniques were an innovation of equestrian nomads of the Central Asian and Iranian steppe and pastoralist tribes such as the Persian Parthians and Sarmatians.
The photograph above left shows Assyrian cavalry from reliefs of 865–860 BC. At this time, the men had no spurs, saddles, saddle cloths, or stirrups. Fighting from the back of a horse was much more difficult than mere riding. The cavalry acted in pairs; the reins of the mounted archer were controlled by his neighbour's hand. Even at this early time, cavalry used swords, shields, and bows. The sculpture implies two types of cavalry, but this might be a simplification by the artist. Later images of Assyrian cavalry show saddle cloths as primitive saddles, allowing each archer to control his own horse.