John Gill Commentary Isaiah 29:8

"And it shall be as when a hungry man dreameth, and, behold, he eateth; but he awaketh, and his soul is empty: or as when a thirsty man dreameth, and, behold, he drinketh; but he awaketh, and, behold, he is faint, and his soul hath appetite: so shall the multitude of all the nations be, that fight against mount Zion." — Isaiah 29:8 (ASV)

It shall be even as when a hungry [man] dreams, and he eats. That is, he dreams of food, and imagines it before him, and that he is really eating it: but he awakes, and his soul is empty.

Or as when a thirsty man dreams, and he drinks. Who fancies that he has got a cup of liquor in his hand, and at his mouth, and is drinking it with a great deal of eagerness and pleasure: but he awakes, and he is faint, and his soul has appetite.

When he awakes, he is not at all refreshed with his imaginary drinking, but still desires liquor to revive his fainting spirits, and extinguish his thirst: so shall the multitude of all the nations be, that fight against Mount Zion; either shall quickly perish; or, having raised their expectations, and pleased themselves with the booty they should obtain, of which they thought themselves sure, shall find themselves mistaken, and all like an illusive dream.

Some interpret this of the disappointment of Sennacherib's army; and others of the insatiable cruelty of the Chaldeans; but rather, if the above sense pleases not, it would be better to understand it of the Jews, who, amidst their greatest danger, flattered themselves with the hope of deliverance, which was all a dream and an illusion; and to which sense the following words seem to incline.