‘You wounded the head from the house of the wicked,
Laying bare the foundation, even to the neck. Selah (consider that!)
You pierced with his own staves the head of his warriors,
They came as a whirlwind to scatter me.
Their rejoicing was to devour the poor in secret.
You trod the sea with your horses,
The heap of mighty waters.'
Parallels with the conquest continue. The head from the house of the wicked can be seen as referring to a petty king or similar. Many were executed by Joshua (Joshua 10:26; Joshua 12:7-24). Some would be executed by striking the neck, presumably with a sword. The piercing with their own staves may well have referred to the Midianites and their allies (Judges 7:22) who slew each other in panic. They had descended like a whirlwind on Israel (Judges 6:4), and met their end mainly at each other's hands. But in the end God will slay all the heads of the wicked.
‘Their rejoicing was to devour the poor in secret.' Those who slay in secret are especially cursed (Deuteronomy 27:24). This suggests that the Midianites may have regularly secretly sought out lone people desperately seeking food in order to kill them out of vindictiveness. It depicts the worst kind of killer.
‘You trod the sea with your horses, the heap of mighty waters.' In Psalms 77:15-20 YHWH is depicted in Habakkuk 3:15 as redeeming His people, and it ends in Habakkuk 3:19 with Him leading His people like a flock by the hand of Moses. But examination of the context demonstrates quite clearly that between these verses there was great divine activity in terms of water, a great storm and His making His way in the sea and His paths through the waters. This can only be vivid language referring to the crossing of the Reed Sea. Thus He trod the seas, the heap of mighty waters, at the Reed Sea. But what of the horses?
No mention is made in accounts of the wilderness journey of Israel as having horses, although it may well be that some important leaders did have one for their own use which they had obtained in Egypt. But in Isaiah 63:13 Yahweh is said to have led His people through the Reed Sea ‘as a horse in the wilderness'. Thus it may be that Habakkuk, knowing this verse from Isaiah and the ideas behind Psalms 77 (if not the Psalm itself) utilised the references in this way, the horses representing the people of Israel who were like a horse in the wilderness. Israel were YHWH's horses.
Alternately it may be seeing the horses of the pursuing Egyptians as YHWH's horses. They rode into the pathway in the Reed Sea with confidence, but found themselves struggling in the mud, and then treading the sea as the waters came down on them, only to perish.
But the most probable significance in view of Habakkuk 3:8 is that it is referring to the horses of the heavenly host (2 Kings 6:17). YHWH is seen at the Reed Sea leading His invisible horses and their riders as they trod the sea and destroyed the Egyptian troops.
So the poem up to this point has been a glorious paean of victory and praise. It has depicted God as controlling nature, as delivering His people, and as showing His might in the earth. It is a full explanation of why He is able to bring about His purposes described in Chapter s 1 & 2.