“See, you are trusting on the staff of this bruised reed, even on Egypt, on which if a man leans it will go into his hand and pierce it. So is Pharaoh, king of Egypt, to all who trust in him.”
If they are trusting in Egypt let them consider how unreliable Egypt was. His words about Egypt would have gained Isaiah's approval. That was just what he thought as well. Egypt was but a battered reed, which if a man used it as his stay, would go into his hand and pierce it. There is here both a reflection on Egypt's comparative weakness (a bruised reed) and on the fact that she tended to let her allies down (piercing the hand that looked for help). In fact, he says, this is what Pharaoh is like, incapable and unreliable, just as he had proved in the past.