A pre-arranged regulated contest between two persons with deadly weapons with the intent of settling a quarrel or vindicating a point of honour. A duel is not self defence or a quarrel. It is not a public duel where two men fight rather than two armies, for instance, David and Goliath or Hector and Achilles. One or both people are killed usually because of some social slight.
Duelling was unknown in the ancient European world. Roman gladiatorial combats were not duels but forms of public entertainment. Early Teutonic allowed private duels occasioned by affronts to one's honour but the real rational was judicial. It was believed that God would not allow the guilty to prevail over the innocent. The practice evolved into customary law in many places. When more sophisticated forms of adjudication evolved the practice of personal combat continued as a way of settling personal disputes. By the 15th century the duel of honour was well established in Spain and France. In the 16th and 17th centuries authorities began legislating against it. In some places the death sentence was imposed on those participating in one and heavy fines on those assisting. Early this century duelling was permitted under the German military code in extreme cases.
Those, moreover, who provoke a private combat or accept one when challenged, deliberately and unnecessarily intend to take a life or at least wound an adversary. Furthermore, divine law prohibits anyone from risking his life rashly, exposing himself to grave and evident danger when not constrained by duty or generous charity. In the very nature of the duel, there is plainly blind temerity and contempt for life. There can be, therefore, no obscurity or doubt in anyone's mind that those who engage in battle privately and singly take upon themselves a double guilt, that of another's destruction and the deliberate risk of their own lives. Finally, there is hardly any pestilence more deadly to the discipline of civil society and perversive to the just order of the state than that license be given to citizens to defend their own rights privately and singly and avenge their honor which they believe has been violated. Fear is not a just excuse for those who accept the challenge of a duel. They are afraid that they will be publicly disgraced as cowards if they refuse. Lastly, the baseness of duelling is so evident, that in our time, despite the approval and patronage of many, legislators have felt bound to repress it by public authority and published penalties. What is so perverse and destructive in this case is that the written laws for the most part are evaded in substance and in deed; and this often happens with the knowledge and silence of those whose duty it is to punish the guilty and see to it that the laws are enforced. Thus it happens that frequently duels are fought and go unpunished, mocking the law. Absurd, certainly, and unworthy of a sensible man is the belief of those who think that civilians are to be prevented from these contests, yet recommend that they be permitted to the military because, they maintain, such experience sharpens military valor. (Papal Writings, Pastoralis Officii, 1891).