Even though destruction is coming around them, and will approach Jerusalem (as first prophesied in verse 12) yet Jerusalem (referred to here as the land of Zion) need not fear. The Lord warns Jerusalem that Assyria will certainly threaten them (lift up his staff, as did the Egyptians before) but that Jerusalem will be saved. The threat will only be for a short while (verse 25.) The Lord notes that his anger will be abated when the destruction turns on Assyria.
It should be remembered, however, that this text also has an eschatological focus. In the context of the end of time, it is not specifically Assyria the historical nation, but Assyria the symbol for the enemies of God that will be beaten down. As with the other dualistic prophecies of Isaiah, these have applicability to two time periods, a historical defeat of Assyria, and the promise of the like fate to the symbolic Assyria when the Messiah comes in his triumphal entry.