In addition to the written rules there exist certain other well-recognised usages and customs that have developed into, and have become recognised as, rules of warfare.

These usages and customs are still in process of development.

The development of the laws and usages of war is determined by three principles (US Army, 1914, p.14):

• First, that a belligerent is justified in applying any amount and any kind of force which is necessary for the purpose of the war; that is, the complete submission of the enemy at the earliest possible moment with the least expenditure of men and money.
• Second, the principle of humanity, which says that all such kinds and degrees of violence as are not necessary for the purpose of war are not permitted to a belligerent.
• Third, the principle of chivalry, which demands a certain amount of fairness in offence and defence and a certain mutual respect between opposing forces.

Reference

US Army. (1914) Rules of Land Warfare. Available from World Wide Web: http://www.loc.gov/rr/frd/Military_Law/pdf/rules_warfare-1914.pdf. [Accessed: 28 September, 2015].