Nahum is using the image of swarms of locusts not to describe a conquering army, but rather the behaviour of Nineveh’s officials. They multiplied and continued their stripping until the invasion came. Then they spread their wings and flew away. Typical of locust behaviour, they settle down when it is cold, but as soon as the sun warms them, they fly away and no trace is left of them (Nahum 3:17). The leaders of Assyria are going to disappear in the heat of battle, abandoning those who follow them.1 Nineveh’s whole system of corruption, greed, and exploitation is going to flee in the face of the Lord’s judgment. Everything that Nineveh thought would make them strong disappears in the face of opposition.
Elsewhere in Scripture swarms of locusts are symbolic of unstoppable and destructive armies. In this verse locusts symbolize weakness and disloyalty. It is best to read this as an intentional reversal that dramatically shows the failure of Assyria’s army and leadership.2
16 You increased your merchants more than the stars of the heavens. The locust spreads its wings and flies away.