Therefore shall the Lord … send among his fat ones leanness. A continuation of Isaiah 10:12, showing what the nature of Assyria's punishment shall be. The prophet expresses it by two images—first, that of a wasting sickness; and secondly, that of a fire.
The first image expresses that gradual decay of national spirit which saps the vital strength of a nation; the second is more suited to denote some external attack under which the weakened nation should succumb.
There are traces, in the later history of Assyria, both of increasing internal weakness through luxury and effeminacy, and of violent external attacks culminating in the combined Median and Babylonian invasion, before which her power collapsed.