The Indo-European horsemen, who came from the steppes of Russia about 4,900 years ago, were not the first in human society to wage war, but they were the first to carry it so widely throughout the known world. War, in contrast to tribal raids, became a possibility only after the Neolithic Revolution, which gave birth to urban societies that used their agricultural surpluses to achieve specialization and division of labour. For the first time in the history of mankind, a specialized military class emerged. These warriors were employed not only to defend their city, but also to conquer neighbouring cities in order to gain extra agricultural land, labour in the form of slaves, fixed capital, and taxes. With these spoils of war the successful warrior state was able to increase its wealth and material standards of living. Archaeological evidence suggests that war began on a sporadic basis at the same time as urban society in the Fertile Crescent about 9,000 years ago; that it became more systematic in Egypt and Sumer at least 5,600 years ago; and by 5,300 years ago it had become a major dynamic strategy in urban societies that were dominated by warrior kings and elite warrior classes. War and conquest were to dominate ancient
civilization for the following four millennia, reaching their apogee in the Roman Empire.