Albert Barnes Commentary Isaiah 29:5

"But the multitude of thy foes shall be like small dust, and the multitude of the terrible ones as chaff that passeth away: yea, it shall be in an instant suddenly." — Isaiah 29:5 (ASV)

Moreover - These verses (Isaiah 29:5, Isaiah 29:7–8) contain a beautiful description of the destruction of the army of Sennacherib. Though they had laid the plan of a regular siege; though the city, in itself, would not be able to hold out against them, and all was alarm and conscious weakness within; yet in an instant the siege would be raised, and the advancing hosts of the Assyrians would all be gone.

The multitude of thy strangers - This refers to the multitude of the strangers that shall besiege you; they are called ‘thy strangers’ because they besieged or oppressed you. The word ‘strangers’ here, as elsewhere, means “foreigners” (see the note at Isaiah 1:7; Isaiah 5:17; Isaiah 14:1; Isaiah 25:2, 25:5; Isaiah 29:5; Isaiah 60:10).

Shall be like small dust - This means light, fine dust that is easily dissipated by the wind.

Of the terrible ones - This refers to the invading, besieging army that is so much the object of dread.

As chaff that passeth away - (see the note at Isaiah 17:13). This image of chaff driven before the wind, to denote the sudden and entire defeat of enemies, is common in the Scriptures (Psalms 1:4; Psalms 35:5; Hosea 13:13).

Yea, it shall be at an instant suddenly - The forces of Sennacherib were destroyed in a single night by the angel of the Lord (Isaiah 37:36; the note at Isaiah 10:12, Isaiah 10:28–34), and the siege of Jerusalem was, of course, immediately raised.