Objection 1. It would seem that there cannot be strife or discord among the angels. For it is written (Job 25:2): "Who maketh peace in His high places." But strife is opposed to peace. Therefore among the high angels there is no strife.
Objection 2. Further, where there is perfect charity and just authority there can be no strife. But all this exists among the angels. Therefore there is no strife among the angels.
Objection 3. Further, if we say that angels strive for those whom they guard, one angel must needs take one side, and another angel the opposite side. But if one side is in the right the other side is in the wrong. It will follow therefore, that a good angel is a compounder of wrong; which is unseemly. Therefore there is no strife among good angels.
On the contrary, It is written (Daniel 10:13): "The prince of the kingdom of the Persians resisted me one and twenty days." But this prince of the Persians was the angel deputed to the guardianship of the kingdom of the Persians. Therefore one good angel resists the others; and thus there is strife among them.
I answer that, The raising of this question is occasioned by this passage of Daniel. Jerome explains it by saying that the prince of the kingdom of the Persians is the angel who opposed the setting free of the people of Israel, for whom Daniel was praying, his prayers being offered to God by Gabriel. And this resistance of his may have been caused by some prince of the demons having led the Jewish captives in Persia into sin; which sin was an impediment to the efficacy of the prayer which Daniel put up for that same people.
But according to Gregory (Moral. xvii), the prince of the kingdom of Persia was a good angel appointed to the guardianship of that kingdom. To see therefore how one angel can be said to resist another, we must note that the Divine judgments in regard to various kingdoms and various men are executed by the angels. Now in their actions, the angels are ruled by the Divine decree. But it happens at times in various kingdoms or various men there are contrary merits or demerits, so that one of them is subject to or placed over another. As to what is the ordering of Divine wisdom on such matters, the angels cannot know it unless God reveal it to them: and so they need to consult Divine wisdom thereupon. Wherefore forasmuch as they consult the Divine will concerning various contrary and opposing merits, they are said to resist one another: not that their wills are in opposition, since they are all of one mind as to the fulfilment of the Divine decree; but that the things about which they seek knowledge are in opposition.
From this the answers to the objections are clear.
Source: Summa Theologica (New Advent)