The geopolitical collapse of the Egyptian empire is vividly portrayed through a striking medical metaphor. God speaks to the prophet, comparing the shattering of Egypt's power to a crushed limb that is denied proper medical care.
The primary approach among commentators links this broken arm to a specific historical event: the crushing defeat of Pharaoh Necho's army by Nebuchadnezzar, King of Babylon, at Carchemish on the Euphrates River. Occurring in the fourth year of King Jehoiakim's reign, this devastating blow stripped Egypt of vast territories and permanently halted its northward expansion toward the land of Israel. The arm symbolizes Pharaoh's immense might [שטיינזלץ]. By breaking one of the king's arms, God left him crippled and severely weakened on the political stage [מלבי״ם, רד״ק, אברבנאל].
The failure of the empire to recover is detailed using the practices of ancient medicine. Normally, treating a broken bone requires setting it firmly in place with a tight binding [רש״י, מצודת ציון, מלבי״ם] and wrapping the fracture in soft cloths or a supportive compress [רש״י, מצודת ציון, רד״ק, שטיינזלץ]. However, because the injury was left open and untreated, Egypt never managed to restore its national honor or avenge its defeat by the Babylonians, leaving its land conquered [מצודת דוד, אברבנאל].
Offering an additional perspective, [מלבי״ם] explains that Pharaoh did attempt to heal and bandage his broken power during his struggles along the Euphrates, but the treatment was fundamentally flawed. Proper healing of a fracture first requires applying medicine to clear away clotted blood before any soft wrapping or tight binding can take place. Because these crucial steps were not performed correctly, the arm remained limp and entirely useless.
The ultimate consequence of this shattered limb is a complete inability to hold a sword. Pharaoh was stripped of his military capacity to fight or even defend himself [רש״י, שטיינזלץ]. Plagued by this enduring weakness, the Egyptian king lost his confidence and never again dared to leave his borders to launch a military campaign [רש״י, רד״ק, אברבנאל].