God will use foreign nations, peoples to bring judgment and chastening to His people.
Calvin on Habakkuk 1:6 "6. For, lo, I raise up the Chaldeans, that bitter and hasty nation, which shall march through the breadth of the land, to possess the dwelling places that are not theirs."

This verse is added by the Prophet as an explanation; for it was not enough to speak generally of God’s work, without reminding them that their destruction by the Chaldeans was nigh at hand. He does not indeed in this verse explain what would be the character of that judgement which he had mentioned in the last verse Habakkuk 1:5; but he will do this in what follows. Now the Prophets differ from Moses in this respect, for they show, as it were by the finger, what he threatened generally, and they declare the special judgements of God; as it is indeed evident from the demonstrative adverb, “Behold.”

How necessary this was, we may gather from the perverseness of that people; for how distinctly soever the Prophets showed to them God’s judgements, so that they saw them with their eyes, yet so great was their insensibility, that they despised denunciations so apparent. What, then, would have been done, if the Prophets had only said in general, ‘God will not spare you!’ This, then, is the reason why the Prophet, having spoken of God’s terrible vengeance, now declares in express terms, that the Chaldeans were already armed by Him to execute His judgement.