Charles Ellicott Commentary Nahum 1:12

"Thus saith Jehovah: Though they be in full strength, and likewise many, even so shall they be cut down, and he shall pass away. Though I have afflicted thee, I will afflict thee no more." — Nahum 1:12 (ASV)

Thus says the Lord. —Better, Thus says Jehovah, Though they are of unimpaired strength and so numerous, yet just in that state they will be cut down, and he [namely, the evil counselor of Nahum 1:11] will pass away. Though I have afflicted you [Jerusalem], I will afflict you no more.

Destruction comes upon the Assyrian army in the very hour of prosperity, while unscathed and complete in numbers (2 Kings 19:32–33).

Pass away: so in Psalm 48:0 (a composition generally thought to refer to this very catastrophe), For lo, the kings were assembled: they passed away together.