Lo, thou trustest - It is possible that Sennacherib might have been apprised 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 ff), 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 doubtless used often for staves, as they are now. They are light and hollow, with long joints. The idea here is, that 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 be attended with injury to themselves (compare Isaiah 30:5, Isaiah 30:7; Isaiah 31:3).