This oracle uses the “arm” metaphor to represent military might. It describes how one of Egypt’s arms had been broken before, but now Babylon will come and break both of Egypt’s arms again. Egypt’s arm was first broken at the battle of Carchemish, an Assyrian city north of Israel and west of Babylon.
This arm was broken again at the siege of Jerusalem when Pharaoh sent an army to assist Judah, but turned back before there was a major confrontation with the Babylonians. Egypt’s status as a major power thus began to wane and Babylon took its place as the most powerful nation in the Near East. The final defeat of Egypt would come later from the Persian empire, but it was as inevitable as the Lord had said.
It is not by their own strength and weakness that empires rise and fall, but by Your will.