Jamieson, Fausset and Brown on Nahum 1:9

9. What do ye imagine against the Lord?--abrupt address to the Assyrians. How mad is your attempt, O Assyrians, to resist so powerful a God! What can ye do against such an adversary, successful though ye have been against all other adversaries? Ye imagine ye have to do merely with mortals and with a weak people, and that so you will gain an easy victory; but you have to encounter God, the protector of His people. Parallel to Is 37:23-29; compare Psa 1:1.

he will make an utter end--The utter overthrow of Sennacherib's host, soon about to take place, is an earnest of the "utter end" of Nineveh itself.

affliction shall not rise up the second time--Judah's "affliction" caused by the invasion shall never rise again. So Na 1:12. But Calvin takes the "affliction" to be that of Assyria: "There will be no need of His inflicting on you a second blow: He will make an utter end of you once for all" (1Sa 3:12; 26:8; 2Sa 20:10). If so, this verse, in contrast to Na 1:12, will express, Affliction shall visit the Assyrian no more, in a sense very different from that in which God will afflict Judah no more. In the Assyrian's case, because the blow will be fatally final; the latter, because God will make lasting blessedness in Judah's case succeed temporary chastisement. But it seems simpler to refer "affliction" here, as in Na 1:12, to Judah; indeed destruction, rather than affliction, applies to the Assyrian.