And prevailed not - Satan and his angels failed in their purpose.
Neither was their place found any more in heaven - They were cast out, and were seen there no more. The idea is, that they were defeated and driven away, though for a time they were suffered to carry on the warfare elsewhere.

And prevailed not - Against the cause of Christianity.
Neither was their place found any more in heaven - The advocates of the heathen idolatry were prevented from having any farther share in the government of the empire. The wonderful success of Constantine over all his enemies, and his final triumph over Licinius, correspond exactly to the symbolical language in this verse.

(15) And prevailed not; neither was their (a) place found any more in heaven.
(15) The description of the victory, by the denying of the thing in this verse, and by affirming the opposite in (Revelation 12:9). As Satan gained nothing in heaven, but was by the power of God thrown down into the world of which he is the prince, Christ himself and his elect members standing still by the throne of God.
(a) They were cast out so, that they were never seen any more in heaven.

And prevailed not,.... That is, the dragon, or the devil, and his angels, prevailed not against Michael and his angels; but, on the other hand, were conquered by them, as the above tyrants were by Constantine and Theodosius: some copies read in the singular number, "and he prevailed not", as the Alexandrian copy, the Complutensian edition the Arabic and Ethiopic versions; and other copies in the plural number, "they prevailed not": and as the Vulgate Latin and Syriac versions:
neither was their place found any more in heaven; in the Roman empire; or "his place", as some copies and versions: this was the time of the judgment of the world, or of the empire as Pagan; Satan the prince of the world, who had long governed in it, was now cast out of all power and authority in it, and all the idol gods in whom he was worshipped, with all the idolatrous priests; nor were there any more any Heathen emperors, for after Constantine's time there was only Julian the apostate, and who reigned but a little while; and after Theodosius, who cleared the empire of Paganism, there never was any, and there is reason to believe there never will be.

prevailed not--A and Coptic read, "He prevailed not." But B and C read as English Version.
neither--A, B, and C read, "not even" (Greek, "oude"): a climax. Not only did they not prevail, but not even their place was found any more in heaven. There are four gradations in the ever deeper downfall of Satan: (1) He is deprived of his heavenly excellency, though having still access to heaven as man's accuser, up to Christ's first coming. As heaven was not fully yet opened to man (John 3:13), so it was not yet shut against Satan and his demons. The Old Testament dispensation could not overcome him. (2) From Christ, down to the millennium, he is judicially cast out of heaven as the accuser of the elect, and shortly before the millennium loses his power against Israel, and has sentence of expulsion fully executed on him and his by Michael. His rage on earth is consequently the greater, his power being concentrated on it, especially towards the end, when "he knoweth that he hath but a short time" (Revelation 12:12). (3) He is bound during the millennium (Revelation 20:1-3). (4) After having been loosed for a while, he is cast for ever into the lake of fire.

And he prevailed not - The dragon himself is principally mentioned; but his angels, likewise, are to be understood. Neither was this place found any more in heaven - So till now he had a place in heaven. How deep a mystery is this! One may compare this with Luke 10:18; Ephesians 2:2; Ephesians 4:8; Ephesians 6:12.