Chemical warfare is the use of non-explosive chemical agents (that are not themselves living organisms, that being biological warfare) to cause injury or death. The main types of agents used in chemical warfare are:

Nerve agents
Mustard agentss
Hydrogen cyanide-based agents
botulinum
Arsines
Toxins
Tear gases
pepper spray
Incapacitating agents such as
Psychotomimetic agents
Potential chemical warfare agents

History
The first major use of chemical warfare agents (weapons) was during World War I, with the use of various agents including chlorine, mustard gas, and phosgene gas by the German army. Other armies quickly responded with chemical weapons of their own. They were not extensively used during World War II due to the fear of retalitation and because chemical weapons are of limited use in a mobile front in which their use would slow the advance of one's own troops. In addition chemical warfare requires supply from railroads which was available in the fixed fronts of World War I, but not the mobile fronts of World War II.
Chemical weapons were also extensively used by both sides during the Iran-Iraq War and are believed to have been used by Iraq against Kurdish civilian populations.

The use of chemical weapons is generally abhored in international law, and there are many rules to discourage or make difficult their acquisition and use. Of these the most important is the Chemical Weapons Convention.

Nuclear, Biological, and Chemical weapons were usually considered morally equivalent and referred to collectively by the phrase "NBC weapons", until this phrase was replaced by weapons of mass destruction, due to confusion about the line between chemical and biological weapons (e.g. prions which are not organisms but simple single-molecule proteins, and could thereby be considered either chemical or biological), concerns about genetic manipulation of biological entities, or nanotechnological methods to generate new molecules with lifelike characteristics, or to exude dangerous chemicals, and the danger of weapons using artificial intelligence and robotics, all of which could conceivably get beyond human control.

By comparison to these threats, the danger of chemical weapons is not considered to be extreme. Even such potential attacks as poisoning of an urban center's water supply (very common in the history of warfare) with a chemical agent, e.g. botulin, are assumed to be containable.

On April 4, 1984 President Ronald Reagan called for an international ban on chemical weapons.

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