The context of this passage in Isaiah is, very quickly, that Judah is looking for strength and security from the Egyptian army rather than consulting God and ultimately looking to him to provide not only a plan but the only real security they can afford in this world. What follows is Isaiah trying to get them to see what they are doing.
Woe to those who go down to Egypt for help and rely on horses, who trust in chariots because they are many and in horsemen because they are very strong, but do not look to the Holy One of Israel or consult the Lord! And yet he is wise and brings disaster; he does not call back his words, but a will arise against the house of the evildoers and against the helpers of those who work iniquity. The Egyptians are man, and not God, and their horses are flesh, and not spirit. When the Lord stretches out his hand, the helper will stumble, and he who is helped will fall, and they will all perish together. (Is 31:1-3)
Listen to what the God of the universe thinks of this:
“Ah, stubborn children,” declares the Lord, “who carry out a plan, but not mine,
and who make an alliance, but not of my Spirit,that they may add sin to sin; who set out to go down to Egypt, without asking for my direction, to take refuge in the protection of Pharaoh and to seek shelter in the shadow of Egypt!(30:1-2)
Notice how trusting in man/flesh is looked on. God not only does not approve it, but it is sin- it is raw disobedience and rebellion (30:1). They are essentially living as though God doesn't exist, as though he didn't love them and long to provide for them. They operate as captains of their own ship, masters of their own fate.
Meanwhile, the Lord sees all and is sovereign (has control over) all. First of all what a slap in the face to God- to have all these promises and to say with your actions, "No, I don't believe it- I don't trust that at all". Second of all, how foolish is it to ignore the invisible but sure security and turn to fallible man.
I've been preaching this to myself while searching for a job. Here's my version of Is 31:
Woe to those who put their sole hope in a high-paying job and who rely on money, who trust in themselves because of their earning power and in their connections because they give them power, but do not look to the Holy One of Israel or consult the Lord! The man thinks he has it all figured out, but going his own way will end in disaster; The Lord does not call back his words, but will set himself against any job or way of making money that is sinful. Employers are man, and not God, and money is flesh, and not spirit. When the Lord stretches out his hand, the bad employer will stumble, and the employee will fall, and they will all perish together. (Altered version of Is 31:1-3)
