War and Peace and War: The Rise and Fall of Empires

by Peter Turchin

Two FACTORS EXPLAIN THE RISE of the Roman Empire: the high degree of internal cohesiveness of the Roman people, or asabiya, which reached a peak c. 200 B.C.; and the remarkable openness of the Romans to the incorporation of other peoples, often recent enemies. Both factors are necessary for building a world empire. Without high asabiya, an incipient imperial nation cannot survive being surrounded by powerful enemies in the early days of its expansion. Without the ability to truly incorporate the conquered people, an imperial nation cannot grow. Theoretically, there is an alternative: genocide or ethnic cleansing of conquered territory followed by colonization. Many imperial nations employed this strategy to some degree, but in practice all successful ones expanded mainly by cultural assimilation, rather than by biological reproduction. Openness to incorporation is a critical factor in explaining the difference between the imperial careers of the Romans and, say, Macedonians, as opposed to the Spartans, or Athenians.