"Behold the handmaid of the Lord: be it done to me according to thy word."

Thus, as the great St. Irenaeus and so many of the holy fathers remark, the obedience of the second Eve repaired the disobedience of the first: for no sooner does the Virgin of Nazareth speak her fiat, 'be it done,' than the eternal Son of God (who, according to the divine decree, awaited this word) is present, by the operation of the Holy Ghost, in the chaste womb of Mary, and there He begins His human life. A Virgin is a Mother, and Mother of God; and it is this Virgin's consenting to the divine will that has made her conceive by the power of the Holy Ghost. This sublime mystery puts between the eternal Word and a mere woman the relations of Son and Mother; it gives to the almighty God a means whereby He may, in a manner worthy of His majesty, triumph over Satan, who hitherto seemed to have prevailed against the divine plan. 

Never was there a more entire or humiliating defeat than that which this day befell Satan. The frail creature, over whom he had so easily triumphed at the beginning of the world, now rises and crushes his proud head. Eve conquers in Mary. God would not choose man for the instrument of His vengeance; the humiliation of Satan would not have been great enough; and therefore she who was the first prey of hell, the first victim of the tempter, is selected to give battle to the enemy. The result of so glorious a triumph is that Mary is to be superior not only to the rebel angels, but to the whole human race, yea, to all the angels of heaven. Seated on her exalted throne, she, the Mother of God, is to be the Queen of all creation. Satan, in the depths of the abyss, will eternally bewail his having dared to direct his first attack against the woman, for God has now so gloriously avenged her; and in heaven, the very Cherubim and Seraphim reverently look up to Mary, and deem themselves honoured when she smiles upon them, or employs them in the execution of any of her wishes, for she is the Mother of their God. 

Therefore is it that we, the children of Adam, who have been snatched by Mary's obedience from the power of hell, solemnize this day of the Annunciation. Well may we say of Mary those words of Debbora, when she sang her song of victory over the enemies of God's people: 'The valiant men ceased, and rested in Israel, until Debbora arose, a mother arose in Israel. The Lord chose new wars, and He Himself overthrew the gates of the enemies.' [Judges v. 7, 8.] Let us also refer to the holy Mother of Jesus these words of Judith, who by her victory over the enemy was another type of Mary: 'Praise ye the Lord our God, who hath not forsaken them that hope in Him. And by me, His handmaid, He hath fulfilled His mercy, which He promised to the house of Israel; and He hath killed the enemy of His people by my hand this night. ... The almighty Lord hath struck him, and hath delivered him into the hands of a woman, and hath slain him.' [Judith xiii. 17. 18; xvi. 7.]

From Dom Prosper Gueranger's commentary on the Annunciation in The Liturgical Year.