423 B.C. Chemical weapons are used in the Peloponnesian War when allies of Sparta take an Athenian-held fort by directing smoke from lighted coals, sulfur, and pitch through a hollowed-out beam into the fort.

Middle Ages The practice of sending dead bodies infected with the plague over city walls during an attack is used.

7th Century The Greeks invent Greek fire, a combination probably of rosin, sulfur, pitch, naphtha, lime and saltpeter. This floated on water and is particularly effective in naval operations.

15th-16th Centuries Venice employs unspecified poisons in hollow explosive mortar shells and sends poison chests to its enemy to poison wells, crops, and animals.

1754 Small pox is used as a biological weapon during the French and Indian Wars by British forces in North America. Soldiers distribute blankets that have been used by smallpox patients with the intent of initiating outbreaks among American Indians. Epidemics occur, killing more than 50 percent of many affected tribes.

1796 Edward Jenner demonstrates that an infection caused by cowpox protects against smallpox. The rapid worldwide use of cowpox inoculation greatly diminishes the potential threat of smallpox as a bio-weapon.

April 15, 1915 At the beginning of World War I, German units release an estimated 150 tons of chlorine gas from some 6,000 cylinders near Ypres, Belgium. This is the first massive use of chemical agents by a warring nation and about 800 troops are killed.

July 12, 1917 Germany tries a new, more deadly gas during the stalemated trench warfare of WWI, releasing sulfur mustard again near Ypres, Belgium. This time, 20,000 troops die. Trench warfare for the remainder of the war would include Germans, French, English and Russians firing chemical agents into opposing enemy troops. Estimates range from 300,000 to 900,000 deaths due to chemical agents in World War I.

After WWI Great Britain allegedly uses chemicals against the Russians and mustard gas against the Afghans north of the Khyber Pass. Spain is said to employ mustard shells and bombs against the Riff tribes of Morocco.

1932 The bacteria that causes tularemia is one of a number of agents studied at Japanese germ warfare research units operating in Manchuria until 1945.

World War II With the possible exception of Japan during attacks on China, no nation during World War II used chemical agents on the battlefield, although Germany employed cyanide and perhaps other chemical agents in its concentration camps.

1963-1967 During the Yemen War, Egypt probably used mustard bombs in support of South Yemen against royalist troops in North Yemen.

Late 1960s The bacteria that cause tularemia is one of several biological weapons stockpiled by the US military.

Vietnam War The United States uses riot-control agents and defoliants, such as Agent Orange, in Vietnam and Laos.

1970 The United States terminates its biological weapons development program by executive order of President Richard M. Nixon and, by 1973, had destroyed its entire biological arsenal.

1970s Although the 1972 Biological and Toxin Weapons Convention prohibited offensive research and production of biological weapons, signatories Iraq and the Soviet Union subsequently produced botulinum toxin for use as a weapon.

Late 1970s Reports of the use of chemical weapons against the Cambodian refugees and against the Hmong tribesmen of central Laos surface, and the Soviet Union is accused of using chemical agents in Afghanistan.

1979 The accidental aerosolized release of anthrax spores from a military micro-biology facility in Sverdlovsk in the former Soviet Union results in at least 79 cases of anthrax infection and 68 deaths and demonstrates the lethal potential of anthrax aerosols.

1980s Widely publicized reports of Iraqi use of chemical agents against Iran lead to a U.N. investigation that confirms the use of the vesicant mustard and the nerve agent tabun. Later during the war, Iraq apparently also uses the more volatile nerve agent sarin. Iran is suspected of using chemical agents to a limited extent in an attempt to retaliate for Iraqi attacks. Press reports also implicate cyanide in the deaths of Kurds in northern Iraq.

1990 Iraq deploys specially designed missiles with a 600-km range; 13 of these are filled with botulinum toxin, 10 with aflatoxin, and two with anthrax spores. Iraq also deploys special 400-lb bombs for immediate use; 100 bombs contain botulinum toxin, 50 contain anthrax spores, and 7 contained aflatoxin. Iraq choses to weaponize more botulinum toxin than any other of its known biological agents.

1991 After the Persian Gulf War, Iraq admits to the United Nations inspection team to having produced 19,000 L of concentrated botulinum toxin, of which approximately 10,000 L were loaded into military weapons. These 19,000 L of concentrated toxin are not fully accounted for and constitute approximately three times the amount needed to kill the entire current human population by inhalation.

1990-1995 Botulism aerosols are dispersed at multiple sites in downtown Tokyo and at US military installations in Japan on at least three occasions between 1990 and 1995 by the Japanese cult Aum Shinrikyo. These attacks failed, apparently because of faulty microbiological technique, deficient aerosol-generating equipment, or internal sabotage.

June 1994 Seven people are killed and 144 others sickened in a sarin gas attack carried out by religious cult, Aum Shinrikyo on a quiet residential area of Matsumoto, Japan.

March 1995 Aum Shinrikyo strikes again, releasing sarin gas in a Tokyo subway, killing 12 people and sickening thousands of others during morning rush-hour.

Sept.-Oct. 2001 More than three dozen people are either infected or exposed to anthrax bacteria originating from pieces of mail in bio-terrorist attacks in Florida, New York, Nevada and Washington, D.C. One death results from the exposures.