A fierce battle between Roman defenders and invading Persians took place at Dura, a garrison city on the Euphrates River in what is now Syria. That was around a.d. 256, nearly seventeen centuries before the 1925 Geneva Protocol outlawed the use of poison or asphyxiating gas in warfare. The ban might have altered the outcome at Dura had it been in force at the time.

Twenty Roman soldiers died quickly in a tunnel when the Persians forced in hot, sulfurous gas, says archaeologist Simon T. James of the University of Leicester in England. The Roman tunnel was intended to head off one that the Persians were digging to undermine a city wall. James points to sulfur crystals and pitch found in the Roman tunnel near its interception of the Persian one. When ignited, the substances produce an asphyxiating gas.

James thinks that after gassing the Romans, the Persians stacked the bodies in a heap, then set their victims' tunnel ablaze with combustibles, including the pitch and sulfur.

The site provides the earliest known archaeological evidence of chemical warfare, says James. Ancient Greek texts describe the use of gas as a weapon, he notes, so its deployment at Dura was not an innovation, but it shows that third-century Persian warriors were more technologically advanced than presumed. They eventually conquered, and later abandoned, Dura, which then lay undisturbed until its rediscovery in 1920.

The findings were presented at the Archaeological Institute of America annual meeting in January.