Chemical and biological weapons have played a part in warfare since ancient times, in spite of the fact that their use generally has been considered repugnant. Poisoning an enemy's wells is an age-old practice. Smoke screens, incendiary devices, and toxic fumes were being used in India some 4,000 years ago. In 1456, a Turkish army attacking Belgrade was driven off by toxic gases the defenders created by burning chemical-soaked rags. In colonial America in 1763, smallpox-infected blankets were sent to the Indians by the British commander Jeffrey Amherst. During the Crimean War at the siege of Sevastopol (1855) a British officer suggested the use of burning sulfur, but the British government rejected the suggestion. During the American Civil War, the United States government similarly rejected a proposal that Union troops use shells containing chlorine.

The Hague Conference of 1899 adopted a resolution banning the use of projectiles containing “asphyxiating or deleterious gases,” which was signed by most of the major powers. This ban lasted only until the next war. The Germans began using poison gas in the first year of World War I (1914) and the Allies in the next. Although gas had no effect on the outcome, it took a terrible toll—nearly 1,000,000 casualties, including nearly 100,000 deaths.

The public outcry against gas led to the Geneva Protocol of 1925, which banned the use in war of lethal gases and biological agents except in retaliation for an enemy's use of them. (More than 80 nations eventually ratified the protocol.) Violations of the Geneva Protocol have been rare. Gas was reported to have been used by the Italians against the Ethiopians in 1936 and by the Japanese against the Chinese, 1937-43. In the 1960's, the Egyptians were charged with using poison gases in Yemen.

The use by the United States in the Vietnamese War of tear gas (to flush enemy troops out of hiding) and of defoliants (to destroy vegetation that might provide sanctuary for the enemy) provoked international protests. A 1969 United Nations resolution affirmed that the use of tear gas and defoliants was a violation of the Geneva Protocol. In 1974 the United States declared it would not use such chemicals in war except in limited circumstances and ratified the protocol, the last major country to do so. (Earlier, in 1969, the United States had renounced the use of biological agents in war and renounced all but defensive use of lethal chemical weapons.)

The Biological Weapons Convention of 1972, which came into force in 1975 after being ratified by the required minimum of 22 countries, bans the development, production, and stockpiling of biological weapons. Both the United States and the Soviet Union were among the ratifying nations.

In the early 1980's, there were charges that the Soviet Union was waging chemical warfare in Afghanistan and was supplying chemical weapons to its Southeast Asian allies, Laos and Vietnam. In the 1980's, Iraq used poison gas against some of its Kurdish citizens and against Iran.

In 1995, a Japanese cult called Aum Shinrikyo released the nerve gas sarin in the Tokyo subway system, killing 12 people and injuring more than 5,000. This incident was the first time a major attack using a chemical weapon was carried out by a nongovernmental group.

The Chemical Weapons Convention, which bans the production, storage, and use of chemical weapons, came into force in 1997. More than 70 countries, including the United States, ratified the treaty.

In 2001, a number of letters containing weapons-grade anthrax spores were mailed to important public figures. Not only were persons in the areas where the letters were opened exposed to the anthrax, but so were workers at postal facilities. More than a dozen persons were infected and five died.