The Neo-Assyrian Empire dominated the Near East from 910 BC to around 610 BC.

In the early 7th century BC, it became one of the first truly international empires. It was a multi-ethnic state composed of many peoples and tribes of different origins.

Its quick decline by the end of the 7th century has puzzled scientists ever since. Most ascribe it to civil wars, political unrest, and the destruction of the capital, Nineveh, by a coalition of Babylonian and Median forces in 612 BC.

Nevertheless, it has remained a mystery why the empire succumbed so suddenly and so quickly.

Dr Adam Schneider of the University of California-San Diego and Dr Selim Adali of the Koc University's Research Center for Anatolian Civilizations in Istanbul, Turkey, argue that factors such as population growth and droughts contributed to the Assyrian downfall (article).

Paleoclimate data show that conditions in the Near East became more arid during the latter half of the 7th century BC.

"At Tecer Lake in the central Anatolian plateau, the presence of a 6--7 cm thick calcite crust in sediment layers dated to 670--630 BC was interpreted as evidence of a short-term, intense episode of desiccation within an otherwise humid phase," the scientists wrote in the paper.

"Stable oxygen isotope data obtained from the sediments of Eski Acigol, a former crater lake also located in the central Anatolian highlands, indicates that a gradual increase of aridity over two or three centuries reached its peak during the mid-7th century BC, which was one of the driest periods in the vicinity for the entire Holocene."

"At Lake Iznik in northwestern Anatolia, a shift towards drier conditions at circa 650 BC was inferred from numerous changes in the sediments."

"Stable oxygen isotope data obtained from authigenic calcites of Lake Zeribar in the central Zagros Mountains of western Iran also indicate that a short-term arid phase, which began in 750 BC, reached its peak intensity during the mid-7th century."

During this time, the region also experienced significant population growth when people from conquered lands were forcibly resettled there.

"It is likely that the population explosion of the heartland area was part of a larger, politically-motivated project of urban expansion undertaken by King Sennacherib (reigned 705--681 BC, son of Sargon II)," the scientists wrote.

"Upon his accession to the throne, Sennacherib decided to move the Assyrian capital to the city of Nineveh. Over the course of his reign, he greatly expanded the city, which by the time of its destruction in 612 BC had grown from 150 ha to a staggering 750 ha in size, making it by far the largest city that had ever existed in northern Mesopotamia up to that time."

"The population of the newly-enlarged capital was greatly increased by the forced immigration of large numbers of deportees to serve as laborers in and around the new imperial metropolis."

The scientists contend that this substantially reduced the state's ability to withstand a severe drought such as the one that hit the Near East in 657 BC. They also note that within five years of this drought, the political and economic stability of the Assyrian state had eroded, resulting in a series of civil wars that fatally weakened it.

"What we are proposing is that these demographic and climatic factors played a significant role in the demise of the Assyrian Empire," Dr Schneider said.