Chemical warfare – ancient Persian-style

The earliest example of chemical warfare has been unearthed at an archaeological site in the Syrian desert, where soldiers of an ancient Persian empire gassed a platoon of Roman troops in about 256AD by asphyxiating them with the smoke from burning bitumen and sulphur.

A makeshift grave of 20 Roman soldiers in full battle armour was discovered at the site of the ancient city of Dura-Europos in the 1930s but it is only now that scientists have been able to figure out exactly how they died.

It was known that they were killed while defending the city against a Persian siege by digging tunnels to counter those being dug by the Sasanian Persian army under the walls of the city. New evidence suggests the Roman troops were deliberately gassed, said Simon James, an archaeologist at Leicester University.

There are no ancient texts that describe the siege of Dura-Europos, which was founded in 300BC by the Macedonian Greeks but its eventual fall to the Sasanians led to its complete abandonment and loss until its rediscovery in the 1920s when the first archaeological excavations began.

Dr James said the Persian siege involved digging mines under the city walls to undermine the fortifications, which would have led to mines being dug by the Roman defenders who would have tried to intercept the Sasanian soldiers in their tunnels.

"It is evident that when mine and counter-mine met, the Romans lost the ensuing struggle," said Dr James. "Careful analysis of the disposition of the corpses shows they had been stacked at the mouth of the counter-mine by the Persians, using their victims to create a wall of bodies and shields. This would have kept the Roman counter-attack at bay while they set fire to the counter-mine, collapsing it, allowing the Persians to resume sapping the walls. This explains whey the bodies were where they were found."

But the question remained: how did the 20 Roman troops die? It would have been difficult to kill too many well-trained and well-armed men in a space less than 6ft high or wide and 36ft long unless something more insidious than brute force was used, Dr James said.

Archaeologists found bitumen and sulphur crystals in the tunnels, which would have been used to get the tunnels burning. But it is also known that these materials give off a highly toxic cocktail of gases when burnt together.

"The Persians would have heard the Romans tunnelling and prepared a nasty surprise for them," said Dr James. "I think the Sasanians placed braziers and bellows in their gallery and when the Romans broke through added the chemicals and pumped choking clouds into the Roman tunnel.

"The Roman assault party were unconscious in seconds, dead in minutes. It is clear from the archaeological evidence at Dura that the Sasanian Persians were as knowledgeable in siege warfare as the Romans. They surely knew of this grim tactic."

The Persians also built a great siege ramp which may have been the deciding factor that led them to overwhelm the Romans and take the city, which was then abandoned, for good.