Assyria had been an enemy of Israel for a long time, but it was time for them to go. Therefore, the Lord promised not to afflict Israel but to afflict Assyria,

Thus says the Lord:
“Though they are safe, and likewise many,
Yet in this manner they will be cut down
When he passes through.
Though I have afflicted you,
I will afflict you no more;
For now I will break off his yoke from you,
And burst your bonds apart.”
(Nahum 1.12–13)

The Assyrian thought he was safe and that his vast numbers would keep him that way, but those things are nothing to the God who allowed that nation to arise.

They had put a yoke on Israel and the Lord would break it. Likewise, the enemies of the church are increasing, but they shall not prevail.