...most people in the West took warfare for granted as an utterly unavoidable part of the social order. Western states fought constantly and devoted most of their disposable resources to this purpose; during the 1700s, no more than six or seven years passed without at least one major European power at war.
The Enlightenment, however, popularized the notion that war was a barbaric relic of mankind's infancy, an anachronism that should soon vanish from the Earth. Human societies, wrote the influential thinkers of the time, followed a common path of historical evolution from savage beginnings toward ever-greater levels of peaceful civilization, politeness and commercial exchange.
The unexpected consequence of this change was that those who considered themselves "enlightened," but who still thought they needed to go to war, found it hard to justify war as anything other than an apocalyptic struggle for survival against an irredeemably evil enemy. In such struggles, of course, there could be no reason to practice restraint or to treat the enemy as an honorable opponent.
Ever since, the enlightened dream of perpetual peace and the nightmare of modern total war have been bound closely to each other in the West. Precisely when the Enlightenment hopes glowed most brightly, wars often took on an especially hideous character.
David A. Bell, "Putting 9/11 In Perspective"