These days, chemical warfare is commonly associated with the modern world, such as the gassing of enemy troops in World War I and the Kurds in 1987-88 by Saddam Hussein's regime in Iraq. But archaeological evidence suggests that it was used in antiquity by Persian warriors when the soldiers suffocated their Roman enemies using bitumen, or pitch (a tarry, sour oil), and sulfur crystals.

Archeologist Simon James of the University of Leicester in England says he discovered a "crime scene" indicating that Persian warriors suffocated 20 Roman soldiers in a Syrian mine. The attack occurred around A.D. 256, when soldiers from the Persian Sassanid Empire invaded Dura-Europos, a highly coveted city on the Euphrates River that had previously been conquered by the Romans.

Although the bodies, stacked atop one another, were discovered in an underground tunnel in the 1930s, James went back to investigate the cause of their deaths. He didn't buy earlier theories that the Romans had died when the tunnel collapsed.

After reviewing archival records showing sulfur crystals and a jar of pitch in the mine, he came up with another theory: The Persians gassed the group to death using highly flammable pitch and sulfur crystals that created sulfur dioxide when the chemicals were burned in a controlled fire. The Persians then piled the corpses on top of one another, using them as a kind of fortress to keep out approaching Romans, and set a large fire to collapse the tunnel.

"We know from the finds that the Persians had bitumen, or pitch, and sulfur crystals—highly inflammable, smoke-generating chemicals," James tells ScientificAmerican.com. "If you'll excuse the pun, it's a smoking gun. Deliberately chucking sulfur crystals onto a fire; in modern terms we call this chemical warfare."

Written texts suggest that the ancient Greeks had engaged in chemical warfare several hundred years earlier, using braziers (pans for holding hot coals), bellows and burning feathers against the Romans in 189 B.C. at the siege of Ambracia. But James says that although this is the earliest archaeological evidence of chemical warfare, no accounts of the battle recorded by participants or witnesses have been unearthed.

His interpretation shows that "you can create a real story out of battlefield patterns that archaeologists find," Melissa Connor, director of forensic science at Nebraska Wesleyan University in Lincoln, told Science News.

James presented his findings last weekend at the Archaeological Institute of America meeting in Philadelphia.

Diagram of proposed gassing of Romans at Dura-Europos, courtesy of Simon James