Early thermal weapons, which used heat or burning action to destroy or damage enemy personnel, fortifications or territories, were employed in warfare during the classical and medieval periods (approximately the 8th century BC until the mid-16th century AD).
Incendiary devices were frequently used as projectiles during warfare, particularly during sieges and naval battles: some substances were boiled or heated to inflict damage by scalding or burning; other substances relied on their chemical properties to inflict burns or damage. These weapons or devices could be used by individuals, thrown by siege engines, or utilised as army strategy. Incendiary mixtures, such as the petroleum-based Greek fire, could be launched by throwing machines or administered through a siphon. Sulfur- and oil-soaked materials were sometimes ignited and thrown at the enemy, or attached to spears, arrow and bolts and fired by hand or machine.
The simplest and most common thermal projectiles were boiling water and hot sand, which could be poured over attacking personnel. Other anti-personnel weapons included the use of hot pitch, oil, resin, animal fat and other similar compounds. Smoke was used to confuse or drive off attackers. Substances such as quicklime and sulfur could be toxic and blinding.
Fire and incendiary weapons were also used against enemy structures and territory, sometimes on a massive scale. Large tracts of land, towns and villages were frequently ignited as part of a scorched earth strategy. Some siege techniques—such as mining and boring—relied on combustibles and fire to complete the collapse of walls and structures.
Towards the latter part of the period, gunpowder was invented, which increased the sophistication of the weapons, starting with fire lances, which led to the eventual development of the cannon and other firearms. Development of the early weapons has continued ever since, with modern war weapons such as napalm, flame throwers, and other explosives having direct roots in the original early thermal weapons. Fire-raising and other destructive strategies can still be seen in modern strategic bombing.
"Fire and sword"
The destruction of enemy possessions and territory was a fundamental strategy of war, serving the dual purpose of punishment and deprivation of resources. Until the 5th century BC, the Greeks had little expertise in siege warfare and relied on a strategy of devastation to draw the enemy out; they destroyed crops, trees and houses. Centuries later, the Byzantines recommended this strategy, even though they had developed siege technology.
Fire was the easiest way of harrying and destroying territories, and could be done easily and quickly by small forces. It was a strategy put to good use by the Scots during the Wars of Independence; they repeatedly launched raids into northern England, burning much of the countryside until the whole region was transformed. King Edward II of England pursued one raiding party in 1327 by following the lights of burning villages.
The tactics were replicated by England during the Hundred Years' War; fire became their chief weapon as they laid waste to the French countryside during lightning raids called chevauchées, in a form of economic warfare. One estimate records the destruction of over 2000 villages and castles during one raid in 1339.
As well as causing the destruction of lands, food and belongings, fire could also be used to divert manpower. 13th century Mongol armies regularly sent out small detachments from their main forces to start grass fires and fire settlements as diversions.
Devastation by fire was not only used as an offensive tactic; some countries and armies employed "scorched earth" policies on their own land to deprive invading armies of all food and forage. Robert I of Scotland reacted to the English invasion of 1322 by launching punitive and diversionary chevauchées into north-west England, then retreating to Culross, burning as he went the Scottish lands which lay in the path of the English army. The English ran out of food and had to abandon the campaign.