Chemical weapons can be widely dispersed in gas, liquid and solid forms, and may easily afflict others than the intended targets.

During World War II the Nazi regime used a commercial hydrogen cyanide blood agent trade-named Zyklon B to commit industrialised genocide against Jews and other targeted populations in large gas chambers.

A lethal agent is designed to injure, incapacitate, or kill an opposing force, or deny unhindered use of a particular area of terrain.

[8] It was not until the 19th century that the modern conception of chemical warfare emerged, as various scientists and nations proposed the use of asphyxiating or poisonous gases.

[10][11] The interwar period saw occasional use of chemical weapons, mainly by multiple European colonial forces to put down rebellions.

Those fears were not unfounded: the Allies made comprehensive plans for defensive and retaliatory use of chemical weapons, and stockpiled large quantities.

[26][27] Terrorist groups have also used chemical weapons, notably in the Tokyo subway sarin attack and the Matsumoto incident.

[30][31] A separate declaration stated that in any war between signatory powers, the parties would abstain from using projectiles "the object of which is the diffusion of asphyxiating or deleterious gases".

[35] This treaty states that chemical and biological weapons are "justly condemned by the general opinion of the civilised world".

The 1993 Chemical Weapons Convention (CWC) is the most recent arms control agreement with the force of International law.

[45] The Oak Ridge National Laboratory conducted a study to assess capabilities and costs for protecting civilian populations during related emergencies,[46] and the effectiveness of expedient, in-place shelters.

[47] At the end of World War II, the Allies occupied Germany and found large stockpiles of chemical weapons that they did not know how to dispose of or deal with.

[48] In the Vietnam War of 1955–1975, a chemical weapon called agent orange was widely used by United States forces.

[49] The United States utilized agent orange as a type of 'tactical herbicide', aiming to destroy Vietnamese foliage and plant life to ease military access.

[49] This usage of agent orange has left lasting impacts that are still observable today in the Vietnamese environment, causing disease, stunted growth, and deformities.

Historically, chemical munitions have been disposed of by land burial, open burning, and ocean dumping (referred to as Operation CHASE).

The Army then began a study of disposal technologies, including the assessment of incineration as well as chemical neutralization methods.

To address growing public concern over incineration, Congress, in 1992, directed the Army to evaluate alternative disposal approaches that might be "significantly safer", more cost effective, and which could be completed within the established time frame.

The Army was directed to report to Congress on potential alternative technologies by the end of 1993, and to include in that report: "any recommendations that the National Academy of Sciences makes ..."[41] In June 2007, the disposal program achieved the milestone of reaching 45% destruction of the chemical weapon stockpile.

[52] The Chemical Materials Agency (CMA) releases regular updates to the public regarding the status of the disposal program.

[56] At the start of World War II it was widely reported in newspapers that "entire regions of Europe" would be turned into "lifeless wastelands".

A German attack on the evening of December 2, 1943, damaged U.S. vessels in the harbour and the resultant release from their hulls of mustard gas inflicted a total of 628 casualties.

[58][59][60] The U.S. Government was highly criticized for exposing American service members to chemical agents while testing the effects of exposure.

[61] Australian service personnel were also exposed as a result of the "Brook Island trials"[62] carried out by the British Government to determine the likely consequences of chemical warfare in tropical conditions; little was known of such possibilities at that time.

Some chemical agents are designed to produce mind-altering changes, rendering the victim unable to perform their assigned mission.

[63] Binary munitions contain two, unmixed and isolated chemicals that do not react to produce lethal effects until mixed.

In contrast, unitary weapons are lethal chemical munitions that produce a toxic result in their existing state.