For the Lord of hosts hath decreed. Isaiah here employs what may be regarded as a concluding exclamation, to confirm more fully the preceding statement. Having said that it is the purpose of the Lord, in order to show that it cannot be broken or made void, (Psalm 33:11,) he puts a question as if about a thing impossible, Who shall disannul his purpose? or, who shall turn back his hand? By this exclamation he speaks disdainfully of all the creatures; for as soon as the Lord has decreed, he stretches out his hand, and when his hand is stretched out, the execution of the work must undoubtedly follow. Nor is it only men whom he declares to be incapable, but he also declares everything else to be incapable of preventing the decree of God; at least if there be anything but man and Satan that opposes his will. In short, he intimates that there can be no repentance or change in God, (Numbers 23:19,) but that whatever may happen, even amidst an endless diversity of events, he continues always to be like himself, and that no occurrence can thwart his purpose. If it be objected that God sometimes changed his purpose, as when he spared the Ninevites, (Jonah 1:2, 3:10,) Abimelech, (Genesis 20:3, 17,) or Pharaoh, (Genesis 12:17,) the answer is easy. When the Lord sent Jonah to the Ninevites, he did not reveal what had been decreed in his secret purpose, but wished to arouse their minds by the preaching of Jonah, that he might have compassion on them. The same thing might be said, when he threatened Abimelech and Pharaoh, because they wished to lay hands on Abraham's wife; for thus the Lord, by terrifying them, intended to keep them back, that they might not suffer the punishment of their obstinacy.

For the Lord of hosts - (see the note at Isaiah 1:9).
Who shall disannul it? - Who has power to defeat his purposes? Difficult as they may be in appearance, and incredible as their fulfillment may seem, yet his purposes are formed in full view of all the circumstances; and there is no power to resist his arm, or to turn him aside from the execution of his designs. By this assurance God designed to comfort his people when they should be in Babylon in a long and dreary captivity (compare Psalm 137:1-9.) And by the same consideration his people may be comforted at all times. His plans shall stand. None can disannul them. No arm has power to resist him. None of the schemes formed against him shall ever prosper. Whatever ills, therefore, may befall his people; however thick, and gloomy, and sad their calamities may be; and however dark his dispensations may appear, yet they may bare the assurance that all his plans are wise, and that they all shall stand. No matter how many, or how mighty may be the foes of the church; no matter how strong their cities, or their ramparts; no matter how numerous their armies, or how self-confident may be their leaders, they have no power to resist God. If their plans are in his way they will be thrown down; if revolutions are necessary among human beings to accomplish His purposes, they will be brought about; if cities and armies need to be destroyed in order that "his" plans may succeed, and his church be safe, they will be demolished, just as the army of Sennacherib was laid pale in death, and as Babylon - the haughtiest of cities - was overthrown. Who can stand against God? and who can resist the execution of his will?

For the Lord of hosts hath purposed,.... What is before declared, the fall of Babylon, and the destruction of the Assyrian, and everything else that comes to pass in this world; there is nothing comes to pass but he has purposed, and everything he has purposed does come to pass:
and who shall disannul it? not the most powerful monarch, or most powerful armies, or the most refined councils of men, or the greatest politicians on earth:
and his hand is stretched out, and who shall turn it back? or aside, from giving the blow it is designed to give; no power on earth is equal to it.

(Daniel 4:35).
To comfort the Jews, lest they should fear that people; not in order to call the Philistines to repentance, since the prophecy was probably never circulated among them. They had been subdued by Uzziah or Azariah (2-Chronicles 26:6); but in the reign of Ahaz (2-Chronicles 28:18), they took several towns in south Judea. Now Isaiah denounces their final subjugation by Hezekiah.