A war policy where no distinction is made between enemy soldiers in action and non-combatants. Black flag policies have the tendency to lead to massacres and other such things that bother modern people; hence, fighting a war this way is against international law now. The best example of a black flag policy can be found in the Old Testament book of Joshua; after he took the city of Jericho, every man, woman and child in the city was killed.

Interestingly, Stonewall Jackson originally wanted to fight in the American Civil War under a black flag policy; he viewed the war as a holy struggle, with the South (obviously) on God's side. (Actually, his view of the war as having a religious nature was far from uncommon, on either side.) Fortunately, cooler heads prevailed on him to obey the usual courtesies regarding keeping prisoners safe and avoiding intentional harm to civilians.