The book of Isaiah is one of the more difficult books to read and understand, simply because people do not pursue a study of the book. Once you pursue a study, then you will quickly learn that this is one of the easiest books to understand, just as the entire Bible is clear and easy to understand, when you engage in a systematic and daily study.
These last two verses, demonstrate the helplessness of man, and the hopelessness of any position that man takes on his own, in this world.
The nation of Israel, both the northern kingdom and the southern kingdom, had turned away from God. The nation as a whole, had abandoned the principles and mandates that were taught in the original Scriptures, and had set out on their own version of the spiritual life, which was completely opposite of that which was mandated by God.
As judgment, God raised up the Assyrian Empire. And evil empire to be sure, however, God often allows the raising up of evil, to confront evil. But sooner or later, that second evil will itself be destroyed, because of its own abuse, failing to see its purpose.
This verse describes the direct intervention of God, in the destruction of Assyria.
It was allowed to advance against Israel, in order to punish Israel. However, as nations go, they fail to see the reality of their purpose in this world and in life, and they themselves failed to turn to God through Christ, which in effect leaves them open to God’s judgment, for their failure.
The phrase, fall by the sword, is typically used to describe a complete destruction.
Not of a mighty man, refers to brilliant tacticians and strategists and military generals.
Not of a mean man, refers to the common soldier.
In other words the defeat of Assyria will not come at the hands of man, but through the direct intervention of God.
Wars and invasions initiated by man, take months and years, to run their course.
However, when God judges, that active judgment comes quickly. The destruction of the Sennacherib army in a single night, sets the precedent for the destruction of all armies worldwide, who will be set against Jerusalem at the end of human history.
And those armies will flee in panic and complete disorganization.
The word for discomfited, describes the attitude of complete fear and terror, and loss of courage.
As those enemy armies are systematically destroyed, so to all of their arrogance and aggressiveness will be taken from them. The word for discomfited, describes the state of nothing being left after taxes and tribute have sapped the wealth and resources of property, leaving it virtually bankrupt.
And so too, the enemies of God, will flee to their own destruction, leaving nothing, when it is all said and done.