"But the Egyptians are mere mortals and not God; their horses are flesh and not spirit. When the Lord stretches out his hand, those who help will stumble, those who are helped will fall; all will perish together." NIV translation
Isaiah warns the people of Jerusalem that, even if the Egyptians decided to try to help them, they would fail because they were up against the will of God and not mere men.
Even as Christians, we often forget that God is in control of all things. We can worry about perceived dangers and even try to form alliances for protection but our hope is in the Lord. Even men with the best of intentions will eventually let us down if we look to them for help.
Isaiah makes it clear that this proposed alliance between Israel and Egypt was an alliance against His will. This is a picture of what will happen in the last days as nations will join together to attempt to overthrow the will of God for Jerusalem.
"This is what the Lord says to me: 'As a lion growls, a great lion over its prey - and though a whole band of shepherds is called together against it, it is not frightened by their shouts or disturbed by their clamor - so the Lord Almighty will come down to do battle on Mount Zion and on its heights. Like birds hovering overhead, the Lord Almighty will shield Jerusalem; he will shield it and deliver it, he will "pass over" it and will rescue it.'" NIV translation
God compares His will with a lion that is feasting on its prey and is threatened by a band of shepherds. Just as men do not scare a big lion from its food, the plans of men to thwart the will of God does not concern Him.
He had promised to protect Jerusalem from the Assyrians and, now, He looks to the final triumph of His will over the feeble efforts of men. He calls to mind the Passover when Israel was spared from death and freed from the bondage of the Egyptians and promises to do the same in the future. This passage speaks of the time just before the start of the Millennial Reign of Jesus Christ.