Right off the bat I'm going to say upfront, I do not hold to the Pre-Trib Rapture theory that began in 1830's Great Britain with John Nelson Darby, et al, and that was then pushed in the Americas by Cyrus Scofield's commissioned study Bible, nor by later pre-trib rapture writers like Hal Lindsey or Tim LaHaye.
Towards the latter part of Revelation 9 (i.e., Rev.9:16 forward), there is mention of a very large army appearing at the 6th Trumpet and 2nd Woe. Those of the Pre-trib Rapture theory especially have taught that means a large physical military army coming upon Israel, and not to worry because Christ will have already 'raptured' His Church off the earth by that time. That whole idea is a falsehood established by the pre-trib theorists to help 'support' their Pre-Tribulational Rapture theory. It is a scare tactic even, to make you want... and to expect... to be raptured prior to the "great tribulation" our Lord Jesus warned of us.
What The Locust Army of Rev.9 Is Really About:
Firstly, one must properly understand the event flow of the last 3 Trumpet, Woe periods. With each one of those last 3 trumpets there is a Woe attached to each one of them. The 5th trumpet - 1st woe begins at the end of Rev.8 and continues half way into Rev.9. Then the 6th trumpet - 2nd woe occurs which is the time of the loosing of that "two hundred thousand thousand" locust army. The 6th trumpet - 2nd woe then continues all the way into Rev.11 and the time of God's two witnesses prophesying for 1260 days, and the Gentiles treading the court of the temple in Jerusalem for 42 months. And then the final 7th trumpet - 3rd last woe occurs after that period. That is the flow our Lord Jesus gave us with those last 3 trumpets with 3 woes.
The battle of Armageddon is aligned with the final 7th trumpet - 3rd woe of Rev.11. That is when all the kingdoms of this world become those of The Father and His Son as written there.
So stop and think a moment about the timing of the previous 6th trumpet - 2nd woe, which begins at Rev.9:12 and continues through to Rev.11:14. That reveals the Armageddon event of Rev.16 is NOT on the 6th trumpet - 2nd woe timing, but on the 7th trumpet - 3rd final woe timing.
So what is that "two hundred thousand thousand" locust army of Rev.9 on the 6th trumpet - 2nd woe? It's certainly not millions of screaming Chinese soldiers.
The 6th trumpet - 2nd woe period is "great tribulation" timing! And it continues all the way up to the 7th trumpet - 3rd woe of Rev.11. That is the kind of time it will be on earth prior to all kingdoms becoming those of The Father and His Christ after that 7th trumpet - 3rd woe of Rev.11:14. It will be the time of "Peace and safety" that Apostle Paul foretold of in 1 Thessalonians 5 that the deceived on earth will saying, and then "sudden destruction" later comes upon them. The "great tribulation" per our Lord Jesus is to be a time of fake peace and safety for the deceived on earth, but a time of tribulation for the saints. This is why our Lord Jesus said in Matthew 24 that we will hear of wars and rumours of wars but don't worry, but the end is not yet. It means as long we are hearing about war and seeing wars, then the time of "great tribulation" is not yet, for war will have ceased on earth for the "great tribulation" timing.
So again, if that "two hundred thousand thousand" army of Rev.9 is not a literal physical military army, what kind of army is it?
Part of the 'key' to understanding about the locusts and that army of Rev.9 is in the prophecy for the end our Heavenly Father gave us through His prophet Joel. In the Book of Joel we are shown God is bringing that great locust army in control upon the earth for the last days, and He gives us military style metaphors with it too, but it is not a military army. The locust symbol and military symbols He uses to show us 'how'... those world controllers work to gain control over the whole earth for the end.
Those locusts represent Satan's host working on earth to prepare whole world for their 'king', the coming Antichrist, to Jerusalem, setting himself up as God, and over all that is called God, or that is worshiped (2 Thess.2). They represent the workers toward a "one world government", the system of the 1st beast kingdom of Rev.13 that Satan will be over during the "great tribulation" timing.
This is why those locusts are told in Rev.9 they cannot hurt any tree, nor any green thing, but only STING. The stinging can only be upon those who are NOT sealed with God's sealing we are also shown in that Rev.9 chapter. The sting itself is towards deception in falling away to worship their coming Antichrist in Jerusalem in place of God. The actual timing for that begins on that 6th trumpet - 2nd woe of Rev.9 when the four angels bound at Euphrates loose those locusts upon the earth for the tribulation (not Armageddon).
Will those locusts be behind the gathering of the great army out of the northern quarters that will come upon Israel for the final battle of Armageddon? Yes. That is specifically what the Rev.16 sixth vial timing is about, with the final 7th vial being the time when they will go upon Israel on the last day of this world. Thus the 7th vial timing also coincides with the 7th trumpet - 3rd woe timing on the last day. And that is when that battle of Armageddon will occur on earth, and will signal our Lord Jesus' return also.
Towards the latter part of Revelation 9 (i.e., Rev.9:16 forward), there is mention of a very large army appearing at the 6th Trumpet and 2nd Woe. Those of the Pre-trib Rapture theory especially have taught that means a large physical military army coming upon Israel, and not to worry because Christ will have already 'raptured' His Church off the earth by that time. That whole idea is a falsehood established by the pre-trib theorists to help 'support' their Pre-Tribulational Rapture theory. It is a scare tactic even, to make you want... and to expect... to be raptured prior to the "great tribulation" our Lord Jesus warned of us.
What The Locust Army of Rev.9 Is Really About:
Firstly, one must properly understand the event flow of the last 3 Trumpet, Woe periods. With each one of those last 3 trumpets there is a Woe attached to each one of them. The 5th trumpet - 1st woe begins at the end of Rev.8 and continues half way into Rev.9. Then the 6th trumpet - 2nd woe occurs which is the time of the loosing of that "two hundred thousand thousand" locust army. The 6th trumpet - 2nd woe then continues all the way into Rev.11 and the time of God's two witnesses prophesying for 1260 days, and the Gentiles treading the court of the temple in Jerusalem for 42 months. And then the final 7th trumpet - 3rd last woe occurs after that period. That is the flow our Lord Jesus gave us with those last 3 trumpets with 3 woes.
The battle of Armageddon is aligned with the final 7th trumpet - 3rd woe of Rev.11. That is when all the kingdoms of this world become those of The Father and His Son as written there.
So stop and think a moment about the timing of the previous 6th trumpet - 2nd woe, which begins at Rev.9:12 and continues through to Rev.11:14. That reveals the Armageddon event of Rev.16 is NOT on the 6th trumpet - 2nd woe timing, but on the 7th trumpet - 3rd final woe timing.
So what is that "two hundred thousand thousand" locust army of Rev.9 on the 6th trumpet - 2nd woe? It's certainly not millions of screaming Chinese soldiers.
The 6th trumpet - 2nd woe period is "great tribulation" timing! And it continues all the way up to the 7th trumpet - 3rd woe of Rev.11. That is the kind of time it will be on earth prior to all kingdoms becoming those of The Father and His Christ after that 7th trumpet - 3rd woe of Rev.11:14. It will be the time of "Peace and safety" that Apostle Paul foretold of in 1 Thessalonians 5 that the deceived on earth will saying, and then "sudden destruction" later comes upon them. The "great tribulation" per our Lord Jesus is to be a time of fake peace and safety for the deceived on earth, but a time of tribulation for the saints. This is why our Lord Jesus said in Matthew 24 that we will hear of wars and rumours of wars but don't worry, but the end is not yet. It means as long we are hearing about war and seeing wars, then the time of "great tribulation" is not yet, for war will have ceased on earth for the "great tribulation" timing.
So again, if that "two hundred thousand thousand" army of Rev.9 is not a literal physical military army, what kind of army is it?
Part of the 'key' to understanding about the locusts and that army of Rev.9 is in the prophecy for the end our Heavenly Father gave us through His prophet Joel. In the Book of Joel we are shown God is bringing that great locust army in control upon the earth for the last days, and He gives us military style metaphors with it too, but it is not a military army. The locust symbol and military symbols He uses to show us 'how'... those world controllers work to gain control over the whole earth for the end.
Those locusts represent Satan's host working on earth to prepare whole world for their 'king', the coming Antichrist, to Jerusalem, setting himself up as God, and over all that is called God, or that is worshiped (2 Thess.2). They represent the workers toward a "one world government", the system of the 1st beast kingdom of Rev.13 that Satan will be over during the "great tribulation" timing.
This is why those locusts are told in Rev.9 they cannot hurt any tree, nor any green thing, but only STING. The stinging can only be upon those who are NOT sealed with God's sealing we are also shown in that Rev.9 chapter. The sting itself is towards deception in falling away to worship their coming Antichrist in Jerusalem in place of God. The actual timing for that begins on that 6th trumpet - 2nd woe of Rev.9 when the four angels bound at Euphrates loose those locusts upon the earth for the tribulation (not Armageddon).
Will those locusts be behind the gathering of the great army out of the northern quarters that will come upon Israel for the final battle of Armageddon? Yes. That is specifically what the Rev.16 sixth vial timing is about, with the final 7th vial being the time when they will go upon Israel on the last day of this world. Thus the 7th vial timing also coincides with the 7th trumpet - 3rd woe timing on the last day. And that is when that battle of Armageddon will occur on earth, and will signal our Lord Jesus' return also.