Habakkuk what was going on the backdrop

Why was it such a surprising thing for God to tell Habakkuk to prophesy of the rise and take over of the Chaldeans via Babylon their seat of power? The Babylonians with help from the Medes sacked Assur the spiritual seat of the Assyrian empire and then Nineveh the very seat of Assyrian power. To this point, Babylon was a proxy kingdom with puppet rulers put in place by the kings of Assyria and posed no credible threat.