the natural jealousy between the members of the Family of Nations, their conflicting interests in many points, and the necessity of a balance of power, are factors of sufficient strength to check the political dangers which such alteration of the ends of a war may eventually involve. [Footnote 78: Ends of war must likewise not be confounded with aims of land and sea warfare; see below, §§ 103 and 173.] [Footnote 79: See Bluntschli, § 536; Lueder in Holtzendorff, IV. p. 364; Rivier, II. p. 219.] [Footnote 80: See above, § 54.] III THE LAWS OF WAR Hall, § 17--Westlake, _Chapters_, pp. 232-235--Maine, pp. 122-159--Phillimore, III. § 50--Taylor, § 470--Walker, _History_, I. §§ 106-108--Heffter, § 119--Lueder in Holtzendorff, IV. pp. 253-333--Ullmann, §§ 167 and 170--Bonfils, Nos. 1006-1013--Despagnet, Nos. 508-510--Pradier-Fodéré, VIII. Nos. 3212-3213--Rivier, II. pp. 238-242--Nys, III. pp. 160-164--Calvo, IV. §§ 1897-1898--Fiore, III. Nos. 1244-1260--Martens, II. § 107--Longuet, p. 12--Bordwell, pp. 100-196--Spaight, pp. 1-19--_Kriegsbrauch_, p. 2--_Land Warfare_, §§ 1-7--Holland, _Studies_, pp. 40-96. [Sidenote: Origin of the Laws of War.] § 67. Laws of War are the rules of the Law of Nations respecting warfare. The roots of the present Laws of War are to be traced back to practices of belligerents which arose and grew gradually during the latter part of the Middle Ages. The unsparing cruelty of the war practices during the greater part of the Middle Ages began gradually to be modified through the influence of Christianity and chivalry. And although these practices were cruel enough during the fifteenth, sixteenth, and seventeenth centuries, they were mild compared with those of still earlier times. Decided progress was made during the eighteenth, and again during the nineteenth century, after the close of the Napoleonic wars, especially in the years from 1850 to 1900. The laws of war evolved in this way: isolated milder practices became by-and-by usages, so-called _usus in bello_, manner of warfare, _Kriegs-Manier_, and these usages through custom and treaties turned into legal rules. And this evolution is constantly going on, for, besides the recognised Laws of War, there are usages in existence which have a tendency to become gradually legal rules of warfare. The whole growth of the laws and usages of war is determined by three principles. There is, first, the principle that a belligerent should be justified in applying any amount and any kind of force which is necessary for the realisation of the purpose of war--namely, the overpowering of the opponent. There is, secondly, the principle of humanity at work, which says that all such kinds and degrees of violence as are not necessary for the overpowering of the opponent should not be permitted to a belligerent. And, thirdly and lastly, there is at work the principle of chivalry which arose in the Middle Ages and introduced a certain amount of fairness in offence