I read this article from an old issue of Review and Herald that I think gives the best explanation of the first part of Joel 2 that I've seen.
The Advent Review and Herald of the Sabbath
March 4, 1875
REMARKS ON THE PROPHECY OF JOEL
VERSE 1. Blow ye the trumpet in Zion, and sound an alarm in my holy mountain: let all the inhabitants of the land tremble: for the day of the Lord cometh, for it is nigh at hand.
The trumpet among the ancient Israelites was to be blown as a token of alarm when danger was near. Was a hostile army approaching, were dangers impending, which required action and haste to escape or avert, the trumpets pealed out an alarm through all the land to arouse the people.
This custom is used as a medium through which to enforce a command upon the servants of God, when the great day of the Lord, in which is summed up for the wicked, all dangers, and all calamities, is about to burst upon the world. Not that a trumpet is now literally to be blown; but an alarm is to be given which shall be as startling, and arrest the attention of the people as effectually, as the trumpet blasts that echoed of old over the mountains of Judea.
The trumpet anciently was to be blown by the priests. Num. 10 : 8. So now the alarm is to be given by the ministers of the Lord. As the priests were the guardians of the people, and were to warn them of approaching danger, so those who are set apart as teachers of God's word, are required to understand the developments of his work in the earth, and to give warning when in the accomplishment of his dispensations, judgments are declared, and to be executed, upon the unfaithful.
The Lord has set his ministers as his watchmen, to see when the sword is coming and warn the people; and by the prophet Ezekiel he charges them with their solemn responsibility in these words:-
"If when he [the watchman] seeth the sword come upon the land, he blow the trumpet and warn the people, then whosoever heareth the sound of the trumpet, and taketh not warning; if the sword come, and take him away, his blood shall be upon his own head. He heard the sound of the trumpet, and took not warning; his blood shall be upon him. But he that taketh warning shall deliver his soul. But if the watchman see the sword come, and blow not the trumpet, and the people be not warned; if the sword come, and take any person from among them, he is taken away in his iniquity; but his blood will I require at the watchman's hand. So thou, O son of man, I have set thee a watchman unto the house of Israel; therefore thou shalt hear the word at my mouth, and warn them from me. When I say unto the wicked, O wicked man, thou shalt surely die; if thou dost not speak to warn the wicked from his way, that wicked man shall die in his iniquity; but his blood will I require at thine hand. Nevertheless, if thou warn the wicked of his way to turn from it; if he do not turn from his way, he shall die in his iniquity; but thou hast delivered thy soul." Eze. 33 : 39.
The trumpet is to be blown in Zion and the alarm sounded in God's holy mountain. A movement is first to be made by the church of God. If the church is asleep what can be expected of the world? The church is set as the light of the world, and should be prompt to see the light and make it known to the people.
But what are the professed churches of our land doing? They have gone down into the plains of Ono, and are joining hands with a pleasure-loving, Judgment-hating, world. And while nature is already beginning to quiver with the throes of its final convulsion, and all over the sea and land around us, and the heavens above us, tokens are multiplying to show that the great day of the Lord is hastening upon us like a lion leaping upon its prey, the church join in the world's mad chase after delusive phantoms, the ministers preach smooth things, and prophesy peace; and the wicked plunging down to death are made to feel quiet and at ease.
Never were there so widespread and confident assertions of a good time coming as at the present day. The cry of peace finds a ready reception in the carnal mind. And since the delusive doctrine of a temporal millennium was invented by Daniel Whitby, about two centuries ago, this sentiment has found such marvelous growth that it is now echoed from almost every pulpit and religious paper in the land.
And what shall be the end thereof? This dream and cry of peace, so securely fostered by the doctrine of the world's conversion, is marked in prophecy as one of the characteristics of the last days, and one of the signs of impending destruction. The day of the Lord, says Paul, comes as a thief in the night. 1 Thess. 5 : 2 And he writes to his "brethren," the true church, that they know this perfectly, bit are not in darkness that the day should overtake them as a thief. Verses 2, 4. Then he says of the world, "For when they shall say, Peace and safety, then sudden destruction cometh upon them." Verse 3.
And this is the time when the prophet, in the chapter before us, calls upon us to blow the trumpet and sound the alarm; for the inhabitants have reason to tremble rather than be at ease, in view of the impending day of the Lord, which shall try every man and every work, with its fiery ordeal.
The next verse sets forth the nature of this day:-
VERSE 2. A day of darkness and of gloominess, a day of clouds and of thick darkness, as the morning spread upon the mountains: a great people and a strong; there hath not been ever the like, neither shall be any more after it, even to the years of many generations.
The world is looking for light; but that which is coming is darkness; they look for joy; but shall find gloominess instead; they look for ease, prosperity, and pleasure, but will suddenly find themselves in clouds and thick darkness; and this day comes like the morning spread upon the mountains, covering all at once.
In the next sentence, as if overwhelmed with the sublimity of the scene before him, with an abrupt exclamation, he introduces the agents by which the events of the day of the Lord are accomplished. "A great people and a strong!" Some mighty army comes forth at the bidding of the Lord. We shall see who they are in subsequent verses.
But does not the reference to future generations destroy the application of this language to the great day of the Lord, beyond which we look for no generations in the ordinary sense of that term? We think not. We take the expression simply to mean that as there had been no day before it like it, there never should be afterward. It stands out conspicuous and alone as a day unparalleled for the sublimity of its scenes, the terror of its judgments, and the magnitude of its revolutions.
The prophet now turns his attention more particularly to the agents concerned in this great work, and devotes the next nine verses to a description of them and their doings.
VERSE 3. A fire devoureth before them; and behind them a flame burneth: the land is as the garden of Eden before them, and behind them a desolate wilderness; yea, and nothing shall escape them. 4. The appearance of them is as the appearance of horses; and as horsemen, so shall they run. 5. Like the noise of chariots on the tops of mountains shall they leap, like the noise of a flame of fire that devoureth the stubble, as a strong people set in battle array. 6. Before their face the people shall be much pained: all faces shall gather blackness. 7. They shall run like mighty men; they shall climb the wall like men of war; and they shall march every one on his ways, and they shall not break their ranks: 8. Neither shall one thrust another; they shall walk every one in his path: and when they fall upon the sword, they shall not be wounded. 9. They shall run to and fro in the city; they shall run upon the wall, they shall climb up upon the houses; they shall enter in at the windows like a thief. 10. The earth shall quake before them; the heavens shall tremble: the sun and the moon shall be dark, and the stars shall withdraw their shining: 11. And 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?
These verses have been the occasion of considerable study: some considering them a description of the devouring locusts mentioned in the first chapter; others, that the language is altogether too strong, and the description altogether too elevated to apply to any insects, or any ravages they have ever been known to commit. Those who apply it to the locusts, as do commentators generally, spend much time in showing their likeness to horses, their sound to that of chariots, their running like men with unbroken ranks, &c. But there are insuperable objections to such an application.
1. When the plague of locusts fell on Egypt, Ex. 10 : 14, it was said that there never should be the like again. If Joel 2 : 2, refers to locusts that came upon Judea in Joel's day, it describes a greater calamity than that of Egypt; for it says that there never had been any like it before; and hence the language of Ex. 10 : 14, is plainly contradicted.
2. The expression, "the day of the Lord," can refer to nothing else but the great day so often referred to in other scriptures. A reference to this day introduces the "great people and strong." described in the verses before us. And the description closes, verse 11, with a reference to the same day. It is most reasonable to conclude that all that intervenes refers to the same time, and the agencies God then makes use of to accomplish his will. It is hardly conceivable that between these plain references to the day of the Lord, Joel should throw in a long description of locusts which in this case devastated Judea centuries ago.
3. The language seems to be entirely too strong for simply a description of the local ravages of a swarm of locusts; especially such expressions as the earth quaking before them, the heavens trembling, and the sun and moon and stars withdrawing their shining.
But the Lord has an army which will be called forth in martial array in the great day of the Lord, to whom the language will apply, and of whose movements and operations the words of the prophet are a striking description; an army that will accomplish the work here specified, and the results of whose presence it is beyond the power of language adequately to portray.
1. The Lord's army. "A great people and a strong," says Joel. Daniel and John in giving the number of this army, which we believe to be none other than the angels of God, say, "Ten thousand times ten thousand [that is one hundred millions] and thousands of thousands." Dan. 7 : 10; Rev. 5 : 11.
2. Will these angels accompany the Lord when he appears in the clouds of heaven? "When the Son of man shall come in his glory, and all the holy angels with him." Matt. 25 : 31.
3. "A fire devoureth before them." "The Lord Jesus shall be revealed from heaven, with his mighty angels, in flaming fire." 2 Them. 1 : 7, 8.
4. "The appearance of them is as the appearance of horses." "And the armies which were in heaven followed him upon white horses, clothed in fine linen white and clean." Rev. 19 : 14. The context shows this to be at the second coming of Christ, and that this is a figure of the angelic hosts which then attend him.
5. "Like the noise of chariots." The Lord "shall appear to your joy, and they shall be ashamed. A voice of noise from the city, a voice from the temple [Rev. 16 : 17], a voice of the Lord that rendereth recompense to his enemies." Isa. 66 : 5, 6. "A noise shall come even to the ends of the earth; for the Lord hath a controversy with the nations, he will plead with all flesh; he will give them that are wicked to the sword, saith the Lord." Jer. 25 : 31. The context shows that this takes place in the day of the Lord.
6. "All faces shall gather blackness." Paleness is the sign of fear and alarm; blackness, of despair. There is no time when "all faces" will gather blackness except in that fearful hour when they realize that probation is no more; that the harvest is past, and the summer is ended, and they are not saved.
7. Verse 7, 8, and 9, reveal some of the characteristics of these heavenly agents whom the Lord at this time commissions to execute his will. No wall can obstruct their march; no bolts nor keys can bar their entrance; their ranks cannot be broken; the sword cannot hurt them; the most secret chamber cannot conceal from them the objects of God's displeasure. Then shall the wicked realize that they are in the hands of executioners, against whom all human safeguard and all human weapons are utterly powerless.
8. Before them the earth quakes, and the sun, moon, and stars withdraw their shining. Joel 3 : 12-15 shows that this is in the day of the Lord; when multitudes are in the valley of decision, when the heathen are gathered to be judged, and the sickle is put in for the final harvest.
9. The Lord utters his voice before his army. The last manifestation the Lord gives to the wicked of his majesty and power, is to utter his voice from heaven just before the appearing of Christ. See Jer. 25 : 30; Joel 3 : 16; Heb. 12 : 26; Rev. 16 : 17.
10. "The day of the Lord is great and very terrible and who can abide it?" Similar language is uttered by the wicked in that day. They exclaim, "The great day of his wrath is come, and who shall be able to stand?" Rev. 6 : 17. None will be able to stand but such as are then clothed in the panoply of truth, and who, while mercy was offered, made the Lord their friend. And they will not only be able to stand, but to them that day will be the rising of the sun of righteousness and the morning of everlasting joy.