Chemical Warfare in 256 AD

Around 256 AD the Persian Sassanid (lately called Sasanian) Empire beseiged the Roman Fortress of Dura, in Syria. Though the Persians never breached the walls, the undersized and unreinforced Imperial contingent eventually fell; everyone within was executed or deported east to live the remainder of their lives as slaves. A recent reexamination of the original 1930s excavation has revealed how: Sassanid sappers digging tunnels under the fort were ready for Roman counterminers tunneling to thwart them from above--when the Romans broke into the Persian tunnels, readied braziers of pitch and sulfur were lit. The resultant rising cloud of sulfur dioxide gas overcame the Roman legionnaires trapped in the dark and congested space in seconds.

This earliest evidentiary use of chemical warfare was not without the consequence that has made it anathema to civilized soldiery throughout subsequent history, however: collateral damage. Along with the bodies of some twenty Roman soldiers excavated in the tunnels were remains of a single Persian soldier...probably the sapper who had set the braziers afire, but miscalculated his escape time and been overcome in the tunnel darkness by his own deadly chemical trap...

•Source: Samir S. Patel, 'Archeology' Jan/Feb 2010, page 26, Archeological Institute of America