Albert Barnes Commentary 2 Kings 18:21

"Now, 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 unto all that trust on him." — 2 Kings 18:21 (ASV)

This bruised reed - The “tall reed of the Nile bulrush” fittingly symbolized the land where it grew. Though appearing strong and firm, it was entirely unworthy of trust. If a person leaned on it, the rotten support would instantly give way, wounding the hand that relied on it so insecurely. This was the case with Egypt throughout the entire period of Jewish history (Compare to 2 Kings 17:4–6). Her actual practice was to feign friendship, hold out hopes of support, and then fail in the time of need.