Emergency workers conduct a training exercise in biochemical gear.
Since their first appearance on the battleground during World War I, chemical weapons have been used almost exclusively as a psychological and diplomatic deterrent. Like nuclear weapons, chemicals are considered weapons of mass destruction. Most defense analysts agree that a chemical attack would certainly provoke retaliation with nuclear weapons, a primary reason why chemical weapons are rarely used. Any nation or political terrorist seeking international support or legitimacy would only provoke outrage if chemical weapons were used. Nevertheless, many rogue nations around the world are believed to be developing them, according to the Defense Department.