Sifting through layers of sediment in Vietnam's Song Hong Delta, researchers weren't surprised to find charred evidence of ancient fires after several cultures migrated there about 5,000 years ago. Cycles of early blazes are tied to changes in the climate, when the area dried out, as well as to agriculture, as a means to clear land for planting. But what could explain a surge in singed land throughout the past 1,500 years?

War, it seems.

A collaborative effort between Vietnamese and Chinese researchers has linked data from the dirt to fashion a new understanding of fire in the area over the past 5,000 years, as humans became a more powerful force in the landscape. They describe their findings in a paper published today in the Proceedings of the National Academy of Sciences.

"In stable societies rulers generally strongly encourage people to farm…. The rulers also supported the building of canals, dams and roads to support irrigation and protect farmlands," the authors note. "Even today, fire is used to clear lands in preparation for agricultural planting in the Song Hong Delta." But these agricultural fires were generally small, local and cyclical.

By studying core samples taken from the river delta, the research team, led by Zhen Li of East China Normal University in Shanghai and colleagues, detected patterns and sizes of charcoal to indicate fire distance (small particles can travel farther than 100 meters, whereas larger ones stay put) as well as pollen to deduce the presence of either crops or nonagricultural flora.

The researchers found that about 1,500 years ago, when the frequency of fires surged, major political unrest between the occupying Chinese and local Vietnamese groups was emergent.

As a military tactic, fire was used to scorch the enemy, destroy supplies, disrupt communication and generally to "create confusion," the authors wrote. One description of a 1788 to 1798 battle between the Chinese and Vietnamese recounts that one warring side surrounded the enemy with fire fueled by rice straw and oil. And the scorched-earth battle tactics continued to ravage the area through the 1900s.

"Fire has played an essential role in the development of human civilization," the authors wrote. But, in this case, it was perhaps more destructive than developmental.