אם שנותי, "When I will whet My flashing blade;" the use of the past tense here when we would have expected a future tense, seeing God speaks about what will be, is typical of prophetic pronouncements. The meaning of the construction is that "if I whet the blade of My sword in the heaven and take hold of judgment on earth." These words parallel those of the prophet (Isaiah 34,5: "for My sword shall be dripping (with blood) in the heaven, lo, it shall come down upon Edom."
A Midrashic approach: "If I were to whet My sword, with a flashing blade, says the Lord, I would destroy the world;" What can I do to prevent this (rhetorical question)? My hand must grasp judgment." So far the words of the Midrash.
You know already that whenever the "sword" of God is mentioned that this is a reference to His attribute of Justice. Examples are Isaiah 66,16: "with His sword against all flesh," or Psalms 2,21: "save my life from the sword," and similar verses in which the word חרב is featured. A man is supposed to constantly be conscious of the impending attack by the attribute of Justice. We have a full verse in Job 19,29 where the author cries out: ""be in fear of the sword, for your iniquity is worthy of the sword. Know there is a judgment." The thrust of the verse is to remind people that what transpires in this universe is not accidental or coincidental, but is part of a master plan." I have already commented on Genesis 3,24 where the flashing blade of the sword guarding the entrance to Gan Eden is mentioned, as well as on Deut. 7,24, that God's sword is extremely sharp, has 16 blades. When the Torah speaks here of חרבי followed by ותאחז, this is an allusion to the 16-sided blade of God's sword by which the wicked will be judged, and which even the righteous have reason to be afraid of; however, (under the appropriate circumstances) this sword is entrusted to the righteous. This is the meaning of the שיר חדש, the new song which will be sung by them in the future, as we know from Psalms 149,6: "with paeans to God in their throats and many-bladed swords in their hands." The sages amend the word בידם, "in their hands," to be read as בי-ה as referring to 16 blades. [The number 15 i.e. י-ה, may refer to the additional number of blades. Ed.]
This song of Moses is a great source of comfort regarding the redemption of the future and the destruction of the idolaters and the avenging of Israel from its enemies, as well as concerning the atonement of Israel's iniquities. Perhaps this is even reflected in the acrostic of the respective first letters of different sections of this song, i.e. the letters ה-ז-י-ו ל-ך, having been so divided by Ezra who arranged the reading into these sections (see beginning of verses 1-7-13-19-29-40). He had foreseen with holy spirit that by reading this portion in the appropriate stanzas there would be an additional ray of hope and an appeal to the people to engage in penitence whenever these verses are read in public, generation after generation
(compare Rosh Hashanah 31). The message of the acrostic is that the radiation of the brilliance of the Shechinah will be restored to us as in former days.
אשיב נקם לצרי, "I shall return vengeance upon My enemies." In other words, I will retaliate against them for what they have done to Israel. God calls the adversaries of Israel: "My enemies," whereas earlier, in verse 27, Moses called them פן ינכרו צרימו, "lest its tormentors would misinterpret." The suffix "its" in that verse referred to Israel. The message of our verse is that anyone who is the tormentor of Israel thereby makes himself into an enemy of the Lord.
You will find a similar construction during the song Moses composed after crossing the Sea of Reeds, where in Exodus 15,7 he speaks of God as "You will destroy those who rise against You, קמיך," where we would have expected Moses to say קמי, "the ones who rose against me." Moses wanted to make the point, however, that anyone attacking the Jewish people is at one and the same time attacking the God of the Jews. This concept is reflected in Psalms 83,3-4 where David says: "for Your enemies rage, Your foes assert themselves. The plot craftily against Your people, take counsel against Your hidden ones." David equates the people who attack Israel with those who rage against the Lord. He refers to the Jewish people as צפונך, "the ones hidden by You," seeing that the Lord hides them, protects them. Actually, the meaning of the word here is close to the meaning of והייתם לי סגולה, "You will be for Me something treasured." One has a tendency of hiding one's treasures. One does not entrust its care to agents, to subordinates.