This portion of the book assumes that the repentance solicited (Joel –17) had occurred and describes the Lord’s zealous response (Joel ,19a) to their prayer. When Israel is in her last extremity, the Lord Jesus will return to rescue the beleaguered Hebrew people from the final conflict at Megiddo.
Joel –21 forms the transition in the message from lamentation and woe to divine assurances of God’s presence and the reversal of the calamities, with Joel b,20 introducing the essence and nature of that reversal. Joel was the first prophet to catch a glimpse of this great climax, which he described in Joel : "The Lord shall utter his voice before his army: for his camp is very great: for he is strong that executeth his word: for the day of the Lord is great and very terrible; and who can abide it?
(The Preacher's Outline & Sermon Bible – Joel, Amos, Obadiah, Jonah, Micah, Nahum) Duane Garrett favors Joel 2 describing a human invasion but links it in with a locust plague description writing that Joel 1 consistently looks to the recent past, but 2:1–11 consistently looks to the future (Many scholars have pointed out the preponderance of perfect forms in chap. It is scarcely reasonable, however, to imagine that Israel had in its liturgical arsenal some kind of “lamentation for future locust plagues.” Both grammar and content demand that a significant shift has taken place: the prophet is looking ahead now, and he sees a human army on the horizon. When he goeth forth to war, famine ravages the land, pestilence smites the nations, hurricane sweeps the sea, tornado shakes the mountains, and earthquake makes the solid world to tremble.
1 in contrast to the shift to the imperfect in chap. Attempts to account for this shift from the viewpoint of a “locust plague” interpretation of Joel 2:1–11 are unsuccessful. This does not mean that the figure of the locust plague has been abandoned entirely in chap. To the contrary, Joel used locust imagery to shape the picture of the invading army. As for animate creatures, they all own his dominion, and from the great fish which swallowed the prophet, down to “all manner of flies,” which plagued the field of Zoan, all are his servants, and like the palmer-worm, the caterpillar, and the cankerworm, are squadrons of his great army, for his camp is very great.
There are other reasons that lead to this conclusion which I discuss on individual verses (e.g., the time phrase in Joel 2:2-note, the "people" and the word "anguish" in Joel 2:6-note). In response, He would remember them Gilbrant - Rûaʿ is used frequently with reference to military activity, to give the call to battle or sound an alarm. On the other hand, this debate is somewhat skewed in that it misses the real point of Joel.
In John Mac Arthur's introductions to the books of the Bible, he always has a section subtitled "Interpretative Challenges." As you might guess, he addresses the difficulties with Joel 2 and follows with helpful notes on the outline of the book... 1 as describing an actual invasion of locusts that devastated the Land. 2, a new level of description meets the interpreter. Trumpets were to be sounded, enlisting the Lord's help prior to battle (Nu 10:9). 15) unto the Lord, and He granted victory at the appearance of the invaders (Joel 2:1). His real concern was not with locusts, or enemy soldiers, or even with the last judgment: the real subject matter of the Book of Joel is the day of the LORD.
In the third section (Joel –), the Lord speaks directly, assuring His people of His presence among them (Joel ; ,21). By that time the land of Israel will have been ravaged by foreign troops and thousands of her people will be dying daily.The whole scene turns military and more than military as its cosmic implications begin to be grasped at Joel –11.(Tyndale OT Commentary) (Bolding Added) Lloyd Ogilvie - Joel uses images from the locust plague, certainly connecting with his audience’s situation, to communicate the much greater danger of the approaching day of the Lord. This offer of grace is similar to the great "BUT GOD" of Ephesians 2 where we read And you were dead in your trespasses and sins, 2 in which you formerly walked according to the course of this world, according to the prince of the power of the air, of the spirit that is now working in the sons of disobedience.Here is more than an army of hungry locusts or the army of another monarch bent on conquest. 3 Among them we too all formerly lived in the lusts of our flesh, indulging the desires of the flesh and of the mind, and were by nature children of wrath, even as the rest.Looking upon the locust swarm, he saw with prophetic insight not just locusts but a mass of human soldiers bearing down on his city, and he described this future army in locust-like terms. My soul, see to it that thou be at peace with this mighty King, yea, more, be sure to enlist under his banner, for to war against him is madness, and to serve him is glory.It would indeed be strange if Joel, prophesying immediately after a locust plague, had described the human army without allowing the locust analogue to influence his language. (New American Commentary - Volume 19A - Hosea-Joel) (Bolding added for emphasis) The description of the plague continues but with a dramatic change: in vivid poetic imagery the prophet compares the locusts to an invading army. Jesus, Immanuel, God with us, is ready to receive recruits for the army of the Lord: if I am not already enlisted let me go to him ere I sleep, and beg to be accepted through his merits; and if I be already, as I hope I am, a soldier of the cross, let me be of good courage; for the enemy is powerless compared with my Lord, whose camp is very great. It is marked, in our Authorized Version, by the word "therefore"; and from Joel to Joel there is an imploring appeal to the nation of Israel to repent before the dread stroke of the Day of the LORD falls.