A more than five-year excavation on the Syrian-Iraqi border has uncovered a sophisticated ancient settlement, suddenly wiped out by invaders 5,500 years ago, that researchers describe as the oldest known excavated site of large-scale organized warfare. The ruined city of Hamoukar, discovered in northeastern Syria, appears to have been a large city by 4,500 BC, said archaeologists Clemens Reichel and Salam al-Quntar, who co-directed Syrian-American excavations on the site. Reichel, a research associate at the Oriental Institute of the University of Chicago, and al-Quntar, of the Syrian Department of Antiquities, jointly announced their latest discoveries Thursday. They said Hamoukar was a flourishing urban center at a time when such cities had been thought to be relegated hundreds of miles (kilometers) to the south. It also appears to be a time when northern and southern cultures clashed in ancient Mesopotamia, a region that includes Iraq and parts of Syria. The site is in the upper edges of the Tigris and Euphrates Valleys, near the Iraq border. Reichel said it may have been settled as long as 8,000 years ago. Scholars long believed urbanized societies began and were isolated in Uruk, in southern Mesopotamia. But excavations at Hamoukar, begun in 1999, and other sites in central Syria led to a reinterpretation of the spread of urban culture in the region. This year, the Syrian-American excavations discovered evidence of the battle that abruptly ended Hamoukar's independence. They found that invaders likely hurled more than 1,200 sling-fired bullets at Hamoukar and more than 100 heavy, 4-inch (10-centimeter) clay balls. The city's walls were toppled and burned. "The whole area of our most recent excavation was a war zone," Reichel said. The ruins preserved not only local pottery and artifacts, but also vast amounts of Uruk pottery. "The picture is compelling," Reichel said. "If the Uruk people weren't the ones firing the sling bullets, they certainly benefited from it. They took over this place right after its destruction." Reichel said archaeologists will have a research advantage if Hamouker's residents were taken by surprise, as their possessions likely are buried with them under the debris.