Archeologists have found the oldest evidence of chemical warfare yet after studying the bodies of 20 Roman soldiers' found underground in Syria 70 years ago.

Clues left at the scene revealed the Persians were lying in wait as the Romans dug a tunnel during a siege – then pumped in toxic gas – produced by sulphur crystals and bitumen – to kill all the Romans in minutes.

Dr Simon James, who solved the mystery, said: "It's very exciting and also quite gruesome. These people died a horrible death.

"The mixture would have produced toxic gases including sulphur dioxide and complex heavy petrochemicals. The victims would have choked, passed out and then died.

"I believe this is the oldest archaeological evidence of chemical warfare ever found. This is the beginning of a particularly nasty history of killing that continues up to the modern day."

Dr James, a researcher at the University of Leicester who presented his discoveries to a meeting of the Archaeological Institute of America, said the 20 soldiers died not by the sword or spear but through asphyxiation.

They had been part of a large Roman garrison defending the empire outpost city of Dura-Europos, on the Euphrates river in modern day Syria, against a ferocious siege by an army from the powerful new Sassanian Persian empire in around AD 256.

There are no historical texts describing the siege but archaeologists have pieced the action together after excavations in the 1920s and 1930s, which have been renewed in recent years.

Evidence shows the Persians used the full range of ancient siege techniques to break into the city, including mining operations to dig under and breach the city walls.

Roman defenders responded with 'countermines' to thwart the attackers. It was in one of these narrow, low galleries that a pile of 20 Roman soldiers was found, apparently stacked up neatly and still with their weapons, in the 1930s.

Dr James returned to the 'cold case' mystery while also conducting new fieldwork at the site in an effort to understand exactly how they died and came to be lying where they were found.

He said: "It is evident that, when mine and countermine met, the Romans lost the ensuing struggle.

"Careful analysis of the disposition of the corpses shows they had been stacked at the mouth of the countermine by the Persians, using their victims to create a wall of bodies and shields, keeping Roman counterattack at bay while they set fire to the countermine, collapsing it and allowing the Persians to resume sapping the walls.

"But this doesn't explain how they died. For the Persians to kill twenty men in a space less than two metres high or wide, and about 11 metres long, required superhuman combat powers – or something more insidious."

Finds from the tunnel revealed that the Persians used bitumen and sulphur crystals to get the fire burning – and this was to prove the vital clue.

Dr James believes the Persians placed braziers and bellows in their gallery, and when the Romans broke through, they added the chemicals to the fire and pumped choking clouds of dense, poisonous gas into the Roman tunnel.

Dr James said: "The Roman assault party were unconscious in seconds, dead in minutes. The Persians must have heard the Romans tunnelling and prepared a nasty surprise for them.

"This is the most likely explanation of how they came to die in such a small space.

"There are ancient history texts that mention Greeks using a technique like this against the Romans, using smoke generators in a tunnel, but this is the first physical evidence of this actually happening.

"One of the surprising things is that people tend to think these eastern empires were not very good at siege warfare.

"But quite clearly the Sassanian Persians were just as good as the Romans."

by:telegraph