While Israel viewed Assyria as its greatest threat, yet as indicated previously in the first two verses, "The Lord will rise up against the house of the wicked, against those who help evildoers." Evil doers and those who aid them, those are the ones the Lord will fight against. There were evil doers in Egypt, Assyria and Israel alike. The "Jerusalem" the Lord fights for is the Jerusalem above, the community of the righteous, and not for the city of Jerusalem, which would be ransacked and its inhabitants taken captive by the Assyrians. For Paul says,  "Now Hagar stands for Mount Sinai in Arabia and corresponds to the present city of Jerusalem, because she is in slavery with her children.  But the Jerusalem that is above is free, and she is our mother."

Furthermore he speaks of Himself as the lion against the shepherds. The shepherds of Israel - its spiritual leaders - had gone astray and become enemies of God, as Isaiah notes later, "His watchmen are blind, They are all ignorant; They are all dumb dogs, They cannot bark; Sleeping, lying down, loving to slumber.Yes, they are greedy dogs Which never have enough. And they are shepherds who cannot understand; They all look to their own way, Every one for his own gain, From his own territory." Is 56:10,11

The shield of birds are likened to the shield of faith mentioned in Ephesians. Its a shield in the realm of ideas. For "We are destroying speculations and every lofty thing raised up against the knowledge of God, and we are taking every thought captive to the obedience of Christ" 2Cor 10:5

Israel is an allegory of Christendom. Is God for or against Christendom? Christendom, which is the present visible kingdom of God, is actually a mix of those who associate themselves with the Christian community. Some are born of God and some are not.  Jesus notes this in a number of his parables including the Parable of the Wheat and the Weeds, the Mustard Seed, the Leaven, the Dragnet, among others. And as he speaks of protecting Jerusalem by "passing over" it, the passover was celebrated with unleavened bread. Leaven has a negative connotation in Jesus' parables. Therefore Paul writes, "Do you not know that a little leaven leavens the whole lump?  Therefore purge out the old leaven, that you may be a new lump, since you truly are unleavened. For indeed Christ, our Passover, was sacrificed for us. Therefore let us keep the feast, not with old leaven, nor with the leaven of malice and wickedness, but with the unleavened bread of sincerity and truth." Therefore let us do so lest we find ourselves to be God's enemies.