"Does the ax raise itself above him who swings it, or the saw boast against him who uses it? As if a rod were to wield him who lifts it up, or a club brandish him who is not wood!" NIV translation
God asks questions of the people to get them to see how ridiculous it is for men to be proud and not give God the credit for the things He has done. We recognize the examples of tools in this verse and realize that they are nothing without the one who powers them. God is reminding them and us that we, too, are nothing and can do nothing without Him and His power.
"Therefore, the Lord, the Lord Almighty, will send a wasting disease upon his sturdy warriors; under his pomp a fire will be kindled like a blazing flame." NIV translation
Because of the pride of their leader, the Assyrian soldiers were to get a disease that would take away their ability to fight.
The king may have trained up the armies but he will be reminded that God is in control. This is a good reminder for us in that we cannot get caught up in pride over things that we have done but instead we must always give God the credit for being the power in us to do anything.
"The Light of Israel will become a fire, their Holy One a flame; in a single day it will burn and consume his thorns and his briers. The splendor of his forests and fertile fields it will completely destroy, as when a sick man wastes away. And the remaining trees of his forests will be so few that a child could write them down." NIV translation
Throughout the Bible fire is a symbol of refining (purifying) things and here we see that the fire of the Lord will cleanse the land of the Assyrians.
We see from the references "Light of Israel" and "Holy One" that this judgment will be by the only one that is qualified to judge and that is Jesus Christ. This day is very similar to the judgment described in Revelation.