From the Epistle of Stephen IV, Otherwise III, to Charles and Carloman, Kings, A.D. 770

The Pontiff strives earnestly to persuade them that neither of them should accept as wife the daughter of Desiderius, king of the Lombards. And indeed, by several arguments he endeavors to show how very unbecoming it would be for Franks to be coupled in marriage to Lombards; but he does not pass over the chief reason from the divine law, although he judged that this should be fortified by other arguments, so that he might more easily deter the minds of those Kings from their purpose. Therefore he says:

"For indeed, most just and most benign kings instituted by God, you have already, by the will and counsel of God, been coupled in legitimate marriage by the precept of your father, accepting, as illustrious and most noble kings, from your same fatherland, namely from that most noble nation of the Franks, most beautiful wives; and it is fitting that you be bound to the love of them. And certainly it is not lawful for you, after dismissing them, to lead other wives, or to mingle yourselves by blood-relationship with an alien nation ... Remember this, most excellent sons, that our predecessor of holy memory, Pope Stephen of most excellent memory, implored your father that he should by no means presume to dismiss your lady and mother; and he, as in very truth a most Christian king, obeyed his salutary admonitions."

If you recall the custom of the Franks (cf. what was said at Canon IX of Vermerie), from which Pepin withdrew only when the Roman Pontiff implored him, you will understand why the Roman Pontiff, for dissuading those nuptials already invalid in themselves, from which nevertheless those Kings did not seem to shrink for political reason, dwells especially in this epistle on developing political arguments.

But Charles the Great did not comply with the Pontiff and accepted as wife the daughter of Desiderius, whom nevertheless after a year he repudiated, "it being uncertain for what cause," says Eginhard; because she was unfit, that is, impotent, says the Monk of St. Gall. There are those who think that the marriage had already been entered before the letters of the Pontiff had been received, and, when these were received, was dissolved by Charles; and for that reason, since those letters were not known to all, Eginhard said: "it being uncertain for what cause." In the same epistle the Pontiff adds other things by which he may deter the Franks from accepting wives from the most base nation of the Lombards. But he seems always to speak on the hypothesis that they were already coupled to another spouse. Read Balan, History of Italy, Book 13, n. 37 and following. Those who defend the cause of Charles in this marriage with the daughter of Desiderius say that he had not yet been coupled to another wife, but only that, from the counsel of his father, another from the nation of the Franks had already been destined, or even betrothed, to him, as also to his brother; and this indeed does not cohere sufficiently with the quoted words of Stephen. It coheres, however, to speak the truth, with the words of Eginhard, who would not have said that it was uncertain for what cause Charles repudiated the daughter of Desiderius, if he had brought her in over a former wife while she was living. Cf. Baronius and Pagi at A.D. 771.