“And the inhabitants of Egypt will know that I am Yahweh, because they have been a staff of reed to the house of Israel. When they took hold of you by the hand, you broke and badly tore their shoulders, and when they leaned on you, you broke, and made all their loins quiver.”
This is the final act which brought down Yahweh's wrath on them, that Egypt had promised to be a strong staff on which Israel could lean, but had turned out to be a mere reed which broke when it was leant on, bringing great harm to Israel. Egypt was in fact a land of reeds, which grew along the Nile and its tributaries, and God says that they were symbolic of what Egypt really was. Thus they must be taught the lesson that they have let down Yahweh's people, and are therefore accountable to Yahweh. God takes constant account of what is done to His people.