A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
isa 31:1THE CHIEF STRENGTH OF THE EGYPTIAN ARMIES LAY IN THEIR CAVALRY. (Isa 31:1-9)
and stay on horses, and trust in chariots--In their level and fertile plains horses could easily be used and fed (Exo 14:9; Kg1 10:28). In hilly Palestine horses were not so easily had or available. The Jews were therefore the more eager to get Egyptian chariots as allies against the Assyrian cavalry. In Assyrian sculptures chariots are represented drawn by three horses, and with three men in them (see Isa 36:9; Psa 20:7; Dan 9:13).
isa 31:2he also is wise--as well as the Egyptian priests, so famed for wisdom (Act 7:22), but who are "fools" before Him (Isa 19:11). He not only devises, but executes what He devises without "calling back His words" (Num 23:19).
home--the whole race.
help--the Egyptian succor sought by the Jews.
isa 31:3not spirit--not of divine power (Psa 56:4; Psa 146:3, Psa 146:5; Zac 4:6).
he that helpeth--Egypt.
isa 31:4(Isa 42:13; Hos 11:10).
roaring on--"growling over" his prey.
abase himself--be disheartened or frightened.
isa 31:5As in the image of "the lion," the point of comparison is the fearless might of Jehovah; so in that of the birds, it is His solicitous affection (Deu 32:11; Psa 91:4; Mat 23:37).
flying--Rather, "which defend" their young with their wings; "to fly" is a secondary meaning of the Hebrew word [MAURER]. "Hovering over" to protect their young [G. V. SMITH].
passing over--as the destroying angel passing over, so as to spare the blood-marked houses of the Israelites on the first passover (Exo 12:13, Exo 12:23, Exo 12:27). He passed, or leaped forward [LOWTH], to destroy the enemy and to spare His people.
isa 31:6The power and love of Jehovah, just mentioned, are the strongest incentives for returning to Him (Eze 16:62-63; Hos 6:1).
ye . . . Israel--The change of person marks that when they return to the Lord, He will address them in more direct terms of communion in the second person; so long as they were revolters, God speaks of them, as more at a distance, in the third person, rather than to them.
isa 31:7In the day of trial the idols will be found to render no help and will therefore be cast away. Compare as to the future restoration and conversion of Israel simultaneously with the interposition of Jehovah in its defense, Zac 12:9-14; Zac 13:1-2.
for a sin--that is, whereby especially you contracted guilt (Kg1 12:30).
isa 31:8Assyrian--Sennacherib, representative of some powerful head of the ungodly in the latter ages [HORSLEY].
sword, not of . . . mighty . . . mean man--but by the unseen sword of God.
flee--Sennacherib alone fled homewards after his army had been destroyed (Isa 37:37).
young men--the flower of his army.
discomfited--rather, "shall be subject to slavery"; literally, "shall be liable to tribute," that is, personal service (Deu 20:11; Jos 9:21) [MAURER]. Or, not so well, "shall melt away" [ROSENMULLER].
isa 31:9Rather, "shall pass beyond his strongholds"; he Shall not stop to take refuge in it through fear (Jdg 20:47; Jer 48:28) [GESENIUS].
ensign--the banner of Jehovah protecting the Jews [MAURER].
fire . . . furnace--"light" and "fire," namely, of Jehovah's altar at Jerusalem (Isa 29:1). Perhaps "furnace," as distinguished from "fire," may mean that His dwelling-place (His hearth) was at Jerusalem (compare Isa 4:5); or else the fiery furnace awaiting all the enemies who should attack Jerusalem.
The times of purity and happiness which shall follow the defeat of the enemies of Jehovah's people (Isa 32:1-8). The period of wrath before that happy state (Isa 32:9-14). The assurance of the final prosperity of the Church is repeated (Isa 32:15-20).