The armies of belligerent States on the outbreak of hostilities, or indeed the moment war is declared, enter into a certain relation with one another which is known by the name of “A State of War.” This relationship, which at the beginning only concerns the members of the two armies, is extended, the moment the frontier is crossed, to all inhabitants of the enemy’s State, so far as its territory is occupied; indeed it extends itself ultimately to both the movable and immovable property of the State and its citizens.

A distinction is drawn between an “active” and a “passive” state of war. By the first is to be understood the relation to one another of the actual fighting organs of the two belligerents, that is to say, of the persons forming the army, besides that of the representative heads of the State and of the leaders. By the second term, i.e., the “passive” state of war, on the other hand, is to be understood the relationship of the hostile army to those inhabitants of the State, who share in the actual conduct of war only in consequence of their natural association with the army of their own State, and who on that account are only to be regarded as enemies in a passive sense. As occupying an intermediate position, one has often to take into account a number of persons who while belonging to the army do not actually participate in the conduct of hostilities but continue in the field to pursue what is to some extent a peaceful occupation, such as Army Chaplains, Doctors, Medical Officers of Health, Hospital Nurses, Voluntary Nurses, and other Officials, Sutlers, Contractors, Newspaper Correspondents and the like.

Now although according to the modern conception of war, it is primarily concerned with the persons belonging to the opposing armies, yet no citizen or inhabitant of a State occupied by a hostile army can altogether escape the burdens, restrictions, sacrifices, and inconveniences which are the natural consequence of a State of War. A war conducted with energy cannot be directed merely against the combatants of the Enemy State and the positions they occupy, but it will and must in like manner seek to destroy the total intellectual38 and material resources of the latter.39 Humanitarian claims such as the protection of men and their goods can only be taken into consideration in so far as the nature and object of the war permit.

Consequently the “argument of war” permits every belligerent State to have recourse to all means which enable it to attain the object of the war; still, practise has taught the advisability of allowing in one’s own interest the introduction of a limitation in the use of certain methods of war and a total renunciation of the use of others. Chivalrous feelings, Christian thought, higher civilization and, by no means least of all, the recognition of one’s own advantage, have led to a voluntary and self-imposed limitation, the necessity of which is to-day tacitly recognized by all States and their armies. They have led in the course of time, in the simple transmission of knightly usage in the passages of arms, to a series of agreements, hallowed by tradition, and we are accustomed to sum these up in the words “usage of war” (Kriegsbrauch), “custom of war” (Kriegssitte), “or fashion of war” (Kriegsmanier). Customs of this kind have always existed, even in the times of antiquity; they differed according to the civilization of the different nations and their public economy, they were not always identical, even in one and the same conflict, and they have in the course of time often changed; they are older than any scientific law of war, they have come down to us unwritten, and moreover they maintain themselves in full vitality; they have therefore won an assured position in standing armies according as these latter have been introduced into the systems of almost every European State.

The fact that such limitations of the unrestricted and reckless application of all the available means for the conduct of war, and thereby the humanization of the customary methods of pursuing war really exist, and are actually observed by the armies of all civilized States, has in the course of the nineteenth century often led to attempts to develop, to extend, and thus to make universally binding these preexisting usages of war; to elevate them to the level of laws binding nations and armies, in other words to create a codex belli; a law of war. All these attempts have hitherto, with some few exceptions to be mentioned later, completely failed. If, therefore, in the following work the expression “the law of war” is used, it must be understood that by it is meant not a lex scripta introduced by international agreements; but only a reciprocity of mutual agreement; a limitation of arbitrary behavior, which custom and conventionality, human friendliness and a calculating egotism have erected, but for the observance of which there exists no express sanction, but only “the fear of reprisals” decides.

Consequently the usage of war is even now the only means of regulating the relations of belligerent States to one another. But with the idea of the usages of war will always be bound up the character of something transitory, inconstant, something dependent on factors outside the army. Nowadays it is not only the army which influences the spirit of the customs of war and assures recognition of its unwritten laws. Since the almost universal introduction of conscription, the peoples themselves exercise a profound influence upon this spirit. In the modern usages of war one can no longer regard merely the traditional inheritance of the ancient etiquette of the profession of arms, and the professional outlook accompanying it, but there is also the deposit of the currents of thought which agitate our time. But since the tendency of thought of the last century was dominated essentially by humanitarian considerations which not infrequently degenerated into sentimentality and flabby emotion (SentimentalitÄt und weichlicher GefÜhlsschwÄrmerei) there have not been wanting attempts to influence the development of the usages of war in a way which was in fundamental contradiction with the nature of war and its object. Attempts of this kind will also not be wanting in the future, the more so as these agitations have found a kind of moral recognition in some provisions of the Geneva Convention and the Brussels and Hague Conferences.

Moreover the officer is a child of his time. He is subject to the intellectual40 tendencies which influence his own nation; the more educated he is the more will this be the case. The danger that, in this way, he will arrive at false views about the essential character of war must not be lost sight of. The danger can only be met by a thorough study of war itself. By steeping himself in military history an officer will be able to guard himself against excessive humanitarian notions, it will teach him that certain severities are indispensable to war, nay more, that the only true humanity very often lies in a ruthless application of them. It will also teach him how the rules of belligerent intercourse in war have developed, how in the course of time they have solidified into general usages of war, and finally it will teach him whether the governing usages of war are justified or not, whether they are to be modified or whether they are to be observed. But for a study of military history in this light, knowledge of the fundamental conceptions of modern international and military movements is certainly necessary. To present this is the main purpose of the following work.