The mentioning of locusts here is significant, indicating that the Assyrians also were “locusts” that devastated Israel (Joel 1:4 and note Joel 1:41). The Assyrian locusts were eventually defeated by the Babylonian locusts.
According to the Minor Prophets, God’s chastising of Israel and His punishing of the nations are for the manifestation of Christ. The main point of Nahum is that while God was chastising Israel and punishing the nations, He always did what was necessary to preserve Israel for the purpose of manifesting Christ. God used one kind of locust to deal with another kind of locust in order to preserve Israel that Christ might have a channel — the reserved and preserved Israel — through which to be brought forth.