The skeleton of a Persian soldier found in the siege tunnels of Dura. The man may have choked on toxic fumes from a fire he himself started. The man's armor is pulled up around his chest; archaeologists suspect he was trying to pull it off as he died.

Almost 2,000 years ago, 19 Roman soldiers rushed into a cramped underground tunnel, prepared to defend the Roman-held Syrian city of Dura-Europos from an army of Persians digging to undermine the city's mudbrick walls. But instead of Persian soldiers, the Romans met with a wall of noxious black smoke that turned to acid in their lungs. Their crystal-pommeled swords were no match for this weapon; the Romans choked and died in moments, many with their last pay of coins still slung in purses on their belts.

Nearby, a Persian soldier — perhaps the one who started the toxic underground fire — suffered his own death throes, grasping desperately at his chain mail shirt as he choked. [Image of skeleton of Persian soldier]

These 20 men, who died in A.D. 256, may be the first victims of chemical warfare to leave any archeological evidence of their passing, according to a new investigation. The case is a cold one, with little physical evidence left behind beyond drawings and archaeological excavation notes from the 1930s. But a new analysis of those materials published in January in the American Journal of Archaeology finds that the soldiers likely did not die by the sword as the original excavator believed. Instead, they were gassed.

Where there's smoke

In the 250s, the Persian Sasanian Empire set its sights on taking the Syrian city of Dura from Rome. The city, which backs up against the Euphrates River, was by this time a Roman military base, well-fortified with meters-thick walls.

The Persians set about tunneling underneath those walls in an effort to bring them down so troops could rush into the city. They likely started their excavations 130 feet (40 meters) away from the city, in a tomb in Dura's underground necropolis. Meanwhile, the Roman defenders dug their own countermines in hopes of intercepting the tunneling Persians.

The outlines of this underground cat-and-mouse game was first sketched out by French archaeologist Robert du Mesnil du Buisson, who first excavated these siege tunnels in the 1920s and 30s. Du Mesnil also found the piled bodies of at least 19 Roman soldiers and one lone Persian in the tunnels beneath the city walls. He envisioned fierce hand-to-hand combat underground, during which the Persians drove back the Romans and then set fire to the Roman tunnel. Crystals of sulfur and bitumen, a naturally occurring, tar-like petrochemical, were found in the tunnel, suggesting that the Persians made the fire fast and hot.

Something about that scenario didn't make sense to Simon James, an archaeologist and historian from the University of Leicester in England. For one thing, it would have been difficult to engage in hand-to-hand combat in the tunnels, which could barely accommodate a man standing upright. For another, the position of the bodies on du Mesnil's sketches didn't match a scenario in which the Romans were run through or burned to death.

"This wasn't a pile of people who had been crowded into a small space and collapsed where they stood," James told LiveScience. "This was a deliberate pile of bodies."

Using old reports and sketches, James reconstructed the events in the tunnel on that deadly day. At first, he said, he thought the Romans had trampled each other while trying to escape the tunnel. But when he suggested that idea to his colleagues, one suggested an alternative: What about smoke?

Fumes of hell

Chemical warfare was well established by the time the Persians besieged Dura, said Adrienne Mayor, a historian at Stanford University and author of "Greek Fire, Poison Arrows & Scorpion Bombs: Biological and Chemical Warfare in the Ancient World" (Overlook Press, 2003).

"There was a lot of chemical warfare [in the ancient world]," Mayor, who was not involved in the study, told LiveScience. "Few people are aware of how much there is documented in the ancient historians about this."

One of the earliest examples, Mayor said, was a battle in 189 B.C., when Greeks burnt chicken feathers and used bellows to blow the smoke into Roman invaders' siege tunnels. Petrochemical fires were a common tool in the Middle East, where flammable naphtha and oily bitumen were easy to find. Ancient militaries were endlessly creative: When Alexander the Great attacked the Phoenician city of Tyre in the fourth century B.C., Phoenician defenders had a surprise waiting for him.

"They heated fine grains of sand in shields, heated it until it was red-hot, and then catapulted it down onto Alexander's army," Mayor said. "These tiny pieces of red-hot sand went right under their armor and a couple inches into their skin, burning them."

So the idea that the Persians had learned how to make toxic smoke is, "totally plausible," Mayor said.

"I think [James] really figured out what happened," she said.

In the new interpretation of the clash in the tunnels of Dura, the Romans heard the Persians working beneath the ground and steered their tunnel to intercept their enemies. The Roman tunnel was shallower than the Persian one, so the Romans planned to break in on the Persians from above. But there was no element of surprise for either side: The Persians could also hear the Romans coming.

So the Persians set a trap. Just as the Romans broke through, James said, they lit a fire in their own tunnel. Perhaps they had a bellows to direct the smoke, or perhaps they relied on the natural chimney effect of the shaft between the two tunnels. Either way, they threw sulfur and bitumen on the flames. One of the Persian soldiers was overcome and died, a victim of his own side's weapon. The Romans met with the choking gas, which turned to sulfuric acid in their lungs.

"It would have almost been literally the fumes of hell coming out of the Roman tunnel," James said.

Any Roman soldiers waiting to enter the tunnels would have hesitated, seeing the smoke and hearing their fellow soldiers dying, James said. Meanwhile, the Persians waited for the tunnel to clear, and then hurried to collapse the Roman tunnel. They dragged the bodies into the stacked position in which du Mesnil would later find them. With no time to ransack the corpses, they left coins, armor and weapons untouched.

Horrors of war

After du Mesnil finished excavations, he had the tunnels filled in. Presumably, the skeletons of the soldiers remain where he found them. That makes proving the chemical warfare theory difficult, if not impossible, James said.

"It's a circumstantial case," he said. "But what it does do is it doesn't invent anything. We've got the actual stuff [the sulfur and bitumen] on the ground. It's an established technique."

If the Persians were using chemical warfare at this time, it shows that their military operations were extremely sophisticated, James said.

"They were as smart and clever as the Romans and were doing the same things they were," he said.

The story also brings home the reality of ancient warfare, James said.

"It's easy to regard this very clinically and look at this as artifacts … Here at Dura you really have got this incredibly vivid evidence of the horrors of ancient warfare," he said. "It was horrendously dangerous, brutal, and one hardly has words for it, really."

You can follow LiveScienceSenior Writer Stephanie Pappas on Twitter @sipappas.