broken the arm of Pharaoh: The Lord had already weakened the Egyptians' power when Nebuchadnezzar defeated Pharaoh Hophra (see Jeremiah 37:5–11). If Hophra had succeeded, Jerusalem would have had temporary relief.
Now, all hope for help from Egypt was gone. There was no chance for the broken arm to heal or even be temporarily supported, so Pharaoh could not protect Jerusalem. Egypt would be completely helpless, unable to wield a sword as it awaited its final defeat.