Yeats writes that Maud Gonne's beauty is so powerful that even Michael, the archangel in charge of God's war with the Devil, would give up his battling for her. Instead, he would devote himself to praising Gonne's beauty peacefully. The example of his peacefulness would finally win the world's sinners over to God's side. God himself gives up his war with Satan, pleased with Gonne's pacifying effect.
This short, simple poem is quite irreverently humorous. It picks up on the suggestion in the final stanza of "The Rose of the World" that Maude Gonne is the most important human being in the world and ought to be worshiped even by archangels.
The archangel Michael is one of the most famous of all heavenly denizens in Catholic traditions. He is the general in charge of God's ongoing war with Satan. The suggestion that this archangel would give up his cause to make a garland for Gonne - like a lovesick undergraduate - is ludicrous and charming. Yeats intends the poem as tongue-in-cheek hyperbole - as a gentle ribbing of Catholic dogma - even as he also intends it as a sincere tribute to the woman he loves above all else in the universe.