Actually, wars against populations and not just between armies began to disappear as a consequence of the end of the Thirty Years' War in the 17th Century. Until that point, armies were largely expected to be self-funding, so they would take resources from local towns (and destroy those of their opposition). Due to the utter exhaustion on all sides at the end of the war, and the cost of supporting large standing armies, a number of military reforms came about, including making armies smaller and more professional, and making non-combatant populations (more or less) off-limits. This is largely were we get the concept of a "civilian" -- someone who has allegiance to a nation but is not directly involved in the fighting of its wars.