DE MONARCHIA OF DANTE ALIGHIERI
CHAPTER X: That which is acquired by single combat is acquired with Right.
1. Whatever is acquired by single combat is acquired with Right. For when human judgment fails, either because it is wrapped in the darkness of ignorance or because it has not the aid of a judge, then, lest judgment should remain forsaken, recourse must be had to Him who so loved her that, by the shedding of His own blood, He met her full demands in death. Hence the Psalm: "The righteous Lord loveth righteousness."  This end is accomplished when, with the free consent of the participants, in love and not in hatred of justice, the judgment of God is sought through a mutual trial of bodily and spiritual strength. Because it was first used in single combat of man to man, this trial of strength we call the duel.
2. But always in quarrels threatening to become matters of war, every effort should be made to settle the dispute through conference, and only as a last resort through battle. Tully and Vegetius both advance this opinion, the former in Moral Duties,  and the latter in his book on The Art of War.  And as in medical treatment everything is tried before final recourse is had to the knife or fire, so when we have exhausted all other ways of obtaining judgment in a dispute, we may finally turn to this remedy by single combat, compelled thereto by the necessity of justice.
3. There are obviously two fixed rules of single combat, one of which we have just now spoken, and another of which we made mention above, that not in hatred, nor in love, but in pure zeal for justice, the contestants or champions should enter the field by common consent. Touching this matter Tully well said: "Wars engaged in for the crown of Empire should be waged without bitterness." 
4. Provided that in single combat these rules are observed without which single combat ceases to be, and that men necessitated by justice and in zeal for justice meet by common consent, are they not met in the name of God? And if they are met in the name of God, is not God in the midst of them, as He Himself promises in the Gospel?  And if God is present, is it not a sin to imagine that Justice  can fail Justice, which we have shown He so greatly loved? And if Justice cannot fail in single combat, is not that which is acquired by single combat acquired by Right?
5. Even before the trumpet-call of the Gospel, the Gentiles recognized this truth, and sought judgment in the fortune of single combat. Pyrrhus, noble in the virtues as well as in the blood of the Aeacidae, answered nobly the legates of the Romans sent to him for redeeming their captives: "I demand no gold, nor shall you render me a price; we are not barterers in war, but fighters; with steel, not with gold, let each decide the issue of life. Whether Hera wills that you or I shall reign, or whatever fate may bring, let us determine by prowess. And at the same time know this: to those whose valor the fortunes of war has preserved, it is my will to grant liberty. Receive them as a gift."  So Pyrrhus spoke, referring by "Hera" to fortune, that agency which we more wisely and rightly name Divine Providence. Let combatants, then, forbear to settle disputes for a price, for that would not be a single combat, but a game of blood and injustice; nor would God then be present as arbiter, but rather that ancient enemy who had been persuader to the quarrel. And let those who desire to be champions, and not hucksters of blood and injustice, have ever before their eyes in entering the field that Pyrrhus who in fighting for Empire, as we have said, held gold in such contempt.
6. If to contradict the truth thus manifested, the usual objection be raised concerning the inequality of men's strength, it may be refuted by the instance of David's victory over Goliath.  And if the Gentiles seek another instance, they may refute it by the victory of Hercules over Antaeus.  It is the height of folly, indeed, to fear that the strength which God confers may be weaker than that of a human antagonist.
7. By this time it is demonstrated clearly enough that whatever is acquired by single combat is acquired with Right and Justice.
1. Ps. 11. 7 (Vulg. 10. 8).
2. De Off. 1. 11. 34.
3. Vegetius, De Re Militari 3. 9. This book on the Art of War is a compilation from many sources, dedicated by its author, of whom nothing is known, to Emperor Valentinian II (375-392). Dante refers to it but this once. This fact, together with Moore's discovery that the context does not bear out the application of the quotation in question, has led Moore to conclude that Dante knew of Vegetius only through a mediaeval handbook or Florilegium. See Moore, Studies, Vol. 1. p. 297.
4. De Off. 1. 12. 38. Church calls attention to the fact that Cicero's word is "Imperii gloria," not "corona."
5. Matt. 18. 20.
6. De Mon. 1. 11.
7. These lines are from Ennius, quoted De Off. 1. 12. 38.
8. 1 Sam. 17. In Letter 7. 6 Dante addresses Henry as a second David come to overthrow a new Goliath.
9. Hercules and Antaeus, used as an example in De Mon. 2. 8. 5. In Inf. 31. 132: "The hands whence Hercules once felt a mighty constraint." The story of the combat is told in detail in Conv. 3. 3. 7.