So which is it? Is God coming to fight Jerusalem or to defend Jerusalem? If I may speak by way of metaphor there are actually two Jerusalems, as Paul alludes saying, "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."
These speak 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. Interesting that in verse 5 he speaks of protecting Jerusalem from birds. In Jesus' parables birds always have a negative connotation. Not being part of the tree, yet they sit in the high branches as if they own the tree and eat its fruits, gobbling up seed which falls to the ground before it has time to germinate. He also 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.
Verse 4 speaks of God as a lion on the prowl. But wait a minute! Doesn't the Bible as speak of the devil also as a roaring lion? "Your enemy the devil prowls around like a roaring lion looking for someone to devour."1Peter 5:8 Just as there are two Jerusalems, there are also two lions. So also in proverbs the wise woman gives an invitation Prov 9:1-6, and so does the foolish one Prov 9:13-18. Those who fail to discern the difference often end up like Saul kicking the goads. (Acts 9:5) While for the Christian Satan is defeatable, being an old worn out toothless lion, God is like the young lion in comparison. He's not simply growling for effect. He means it and He's capable of doing it and has no fear.