Albert Barnes Commentary Isaiah 36:6

"Behold, thou trustest upon the staff of this bruised reed, even upon Egypt, whereon if a man lean, it will go into his hand, and pierce it: so is Pharaoh king of Egypt to all that trust on him." — Isaiah 36:6 (ASV)

Lo, thou trustest - It is possible that Sennacherib might have been informed of the attempt which had been made by the Jews to secure the cooperation of Egypt (see the notes at Isaiah 30:1-7; Isaiah 31:1 and following), though he might not have been aware that the negotiation was unsuccessful.

In the staff of this broken reed - The same comparison of Egypt with a broken reed, or a reed which broke while they were trusting to it, occurs in Ezekiel 29:6-7. Reeds were undoubtedly used often for staffs, as they are now. They are light and hollow, with long joints. The idea here is that just as a slender reed would break when a man leaned on it, and would pierce his hand, so it would be with Egypt. Their reliance would give way, and their trusting to Egypt would result in injury to themselves (Compare Isaiah 30:5, Isaiah 30:7; Isaiah 31:3).