There are four primary classes of Lethal Agents that are produced as weapons. There are also two classes of Non-Lethal Harassing Agents.
Lethal agents are classified by the affect that they have on the body, as all have the capacity to cause death.
Harassing agents are less developed, as the line between incapacitation and lethality is often difficult to control with gaseous weapons, and incapacitation was seen as having little use in warfare for most of history.
The categories of chemicals are also unrefined, and the difference is only how used they are - “Incapacitators” are currently only in very rare experimental situations, and have not had good effects or publicity in any of the known situations they were in. ”Riot Control” chemicals are considered non-lethal lachrymatory or vomit agents, and are intended for use against civilians or civilian groups seen as belligerent.
Though chemical weapons became well-known and feared during the First World War, evidence of their use goes back almost 15,000 years, to the South African San tribes, which utilized poison from scorpions and snakes on their arrow tips used for hunting.
However, unlike the Mesoamerican peoples who were shown to have used curare vine and dart frog poison on their weapons several thousand years later, there is a lack of evidence that the San actively used these tactics against other humans. That’s not to say that they didn’t, but there is as yet no solid evidence of it that has been found.
Chemicals in Early Warfare
Despite the efficacy of poison-tipped arrows and weapons in killing individuals, to utilize chemical weaponry in warfare required being able to disperse a substance over a significant area, so as to incapacitate or kill large numbers of enemies. Aside from large-scale water and foodstore poisonings, gaseous compounds have been used against enemies and revolting subjects alike, since before the days of Sun-Tzu in the East, and the Peloponnesian War in the West.
Thucidydes wrote in 500 BCE, in History of the Peloponnesian War, of Spartans burning wood, pitch, and sulfur under the walls of Athens, hoping to incapacitate the enemy before the direct assault. Unfortunately, a thunderstorm rolled in shortly after the incendiaries were ignited, and the tactic failed. There were recorded incidents of burning sulfur in several wars in the following millennium, however, and it functioned as a moderate choking agent when it dispersed correctly.
Recent excavations of Dura-Europos, in modern-day Syria, proved that burning sulfur could be extremely effective in closed quarters, especially when combined with other compounds. In 256 CE, the city launched a counter-attack against Roman forces tunneling under the battlements. The Persians heard the tunneling, and formed a smaller tunnel that connected to the Romans. At the bottom of their tunnel, they lit a fire of pitch, sulfur, and bitumen. The smoke from this fire traveled up a small chimney and into the larger Roman tunnel.
Almost 17 centuries later, excavators in the 1930s would find a pile of twenty men within the large tunnel, with Roman armor, and no apparently mortal wounds to their bones, unlike those found from the same era and area. Though ancient texts proposed chemical and incendiary tactics, proof of their use had never before been found in this area. However, in 2008, a team of archaeologists from the University of Leicester tested the tunnels, remains, and findings within the tunnels, and their findings demonstrated the earliest archaeological proof of chemical warfare.