Scope of the treatise

Controversies among those who are not held together by a common bond of municipal law are related either to times of war or to times of peace. Such controversies may arise among those who have not yet united to form a nation, and those who belong to different nations, both private persons and kings; also those who have the same body of rights that kings have, whether members of a ruling aristocracy, or free peoples.

War, however, is undertaken in order to secure peace; and there is no controversy which may not give rise to war. In undertaking to treat the law of war, therefore, it will be in order to treat such controversies, of any and every kind, as are likely to arise. War itself will finally conduct us to peace as its ultimate goal.

Definition of war, and origin of the word

As we set out to treat the law of war, then, we ought to see what is war, which we are treating, and what is the law which forms the subject of our investigation. Cicero defined war as a contending by force. A usage has gained currency, however, which designates by the word not a contest but a condition; thus war is the condition of those contending by force, viewed simply as such. This general definition includes all the classes of wars which it will hereafter be necessary to discuss.