The Assyrian king might think that it is his own power behind this invasion. But Jehovah declares that after his usefulness has ended, Jehovah will punish that king. Verses 13 and 14 are in the voice of the Assyrian king, boasting of his own strength and wisdom.
In verse 15, Jehovah makes it clear that the Assyrian king is only a tool for Jehovah’s righteous wrath. Jehovah declares: “shall the ax boast itself against him that heweth therewith?” Is the tool able to boast that it is better than the one who wields the tool? Assyria is only the tool, and the usefulness of that tool will have an end. There will be a time when, as verse 12 noted, Jehovah will have performed his whole work against Jerusalem.