Does 2 Thessalonians 1:9-10 contradict premillennialism?

As a classical / post-trib premillenial, I'd point at the preceding verses and note that those that are sent to hell (eternal destruction) are the persecutors of the righteous.

They get killed when Jesus returns, after which the millenial reign commences, then there's a final rebellion and the Lake of Fire.

Going to hell doesn't begin at the Last Judgement, but at the preceding Second Coming.

Where's the problem? Or is this only aimed at pre-trib premillenials?

I suspect the answer to your question depends on how long people think the Day of Judgment lasts. Some say it lasts for a literal thousand years.

Jehovah’s Witnesses, for example, believe that after the Son of Man returns with all the angels, and begins to reign for one thousand years, all who have died will be resurrected with physical bodies to live on a paradise earth. During this time they may be saved. At the end of the thousand years Satan is released. There is a final battle when humans who align with Satan are annihilated (which means a second physical death). This view differs from typical premillennialism in that Jehovah’s Witnesses believe Christ does not physically return to earth but reigns from heaven along with the 144,000 over the inhabitants of a paradise earth. https://www.jw.org/en/library/magazines/g201001/what-is-judgment-day/

However, they also believe that during the great tribulation there will be a judgment on all humans who are enemies of Christ and this is when 2 Thessalonians 1:9 is fulfilled. All who are judged as “goats” will experience everlasting destruction:

“When the Son of man [Jesus Christ] comes in his glory, and all the angels with him, then he will sit down on his glorious throne. All the nations will be gathered before him, and he will separate people one from another, just as a shepherd separates the sheep from the goats. And he will put the sheep on his right hand, but the goats on his left.” This time of judgment will be part of a “great tribulation” unlike anything in human history. That tribulation will culminate in the war of Armageddon. (Matthew 24:21; Revelation 16:16) Christ’s enemies, described in his illustration as goats, “will undergo the judicial punishment of everlasting destruction.” (2 Thessalonians 1:9; Revelation 19:11, 15) In contrast, his faithful servants, the sheep, will have the prospect of “everlasting life.”—Matthew 25:46. https://www.jw.org/en/bible-teachings/questions/jesus-coming/

Here we have two judgments – those deemed enemies of Christ at his return, then a final judgment at the end of the thousand year millennial reign of Christ.

Another premillennialist view is that Christ’s second coming will occur prior to His millennial kingdom, which is a literal 1000-year reign of Christ on earth. During this time Christ will judge the nations on earth but the last judgment will not happen till the end of the millennium.

• Christ’s second coming will occur prior to His millennial kingdom being established on earth

• The final judgment takes place after the millennial reign of Christ at the end of human history and prior to the start of the eternal state

• This final judgment of unbelievers occurs at the end of the Millennium, before the creation of the new heaven and earth. At this judgment, unbelievers from all the ages are judged for their sins and consigned to the lake of fire.

Matthew 25:31-46 is quoted in support of this view: >When the Son of Man comes in his glory, and all the angels with him, then he will sit on his glorious throne. Before him will be gathered all the nations, and he will separate people one from another as a shepherd separates the sheep from the goats.

Revelation 20:7-15 is also used to support the view that the final judgment of unbelievers occurs at the end of the Millennium when unbelievers from all the ages are judged for their sins and consigned to the lake of fire.

What we have here is a day of judgment (separation of the sheep from the goats) that will last for 1,000 years, after which comes the final judgment of unbelievers who face everlasting destruction.

This premillennial view supports the view that Judgment Day happens at the end of time when everyone will stand before God and He will render final judgment on their lives. The final judgment will occur at the end of time. Revelation 19:17–21 records a great battle in which the enemies of God are slaughtered but this is only a temporal judgment on the people alive at the time of the great battle. The final judgment will encompass everyone who has ever lived and will consign people to their final destiny.

The judgment of Judgment Day is carried out according to what individuals have done in their lives—they are judged according to their works. A person will not be judged according to what others did or didn’t do; he stands judgment alone, responsible for his own actions.

The person who has faith in Christ has already had judgment rendered. That person has been justified—that is, declared righteous—by God on the basis of Christ’s perfect work on his behalf. It is as if the final judgment that would have happened on Judgment Day has been rendered in advance. All who have faith in Christ are declared righteous, and their names are written in the Lamb’s Book of Life. They have nothing to fear on Judgment Day because their punishment has already been borne by Christ on the cross (Romans 8:1). For those who have faith in Christ, Judgment Day will be the day of final salvation when they are rescued from all of the adverse effects of sin (Malachi 4:2–3). https://www.gotquestions.org/Judgment-Day.html

The problem, as you point out, is 2 Thessalonians 1:9-10. Speaking to Christian believers about the righteous judgment of God at the revelation from heaven of the Lord Jesus, Paul gives this warning to those who have persecuted the saints and who do not obey Christ Jesus:

Who shall be punished with everlasting destruction from the presence of the Lord, and from the glory of his power; When he shall come to be glorified in his saints, and to be admired in all them that believe (because our testimony among you was believed) in that day (KJV).

When is this revelation from heaven of the Lord Jesus? Scripture clearly states that this Day is when Christ Jesus is revealed from heaven in blazing fire, accompanied with his powerful angels. At this second coming he will be seen by every human on the planet, even those who deny his existence and refuse to recognise him. He comes to punish and all who reject him with everlasting destruction.

When does “the day of the Lord” occur? This expression is found in 1 Thessalonians 5:2 and in 2 Thessalonians 2:1 it is expressed as “the coming of our Lord Jesus Christ”. One source says that some scholars believe that the day of the Lord will be a longer period of time than a single day—a period of time when Christ will reign throughout the world before He cleanses heaven and earth in preparation for the eternal state of all mankind. Other scholars believe the day of the Lord will be an instantaneous event when Christ returns to earth to redeem His faithful believers and send unbelievers to eternal damnation.

You ask: does Paul place Christ's Second Coming within the Day of Judgment? Not according to premillennialists.

You also ask if 2 Thessalonians 1:9-10 connects the Second Coming with the final or last Judgment. According to what I’ve read, premillennialists believe the second coming precedes the thousand-year millennial reign of Christ. During this time will be a period of judgment that culminates in the final destruction of humans who rebel against God and His chosen people after Satan is briefly released.

As already expressed, there are different views from premillennialists as to what happens and when. I doubt you will get a clear answer. If premillennialists have an explanation with regard to 2 Thessalonians 1:9-10 I have yet to find it.

Caveat: I feel obliged to say that God’s judgments are always fair and so I am content to leave it up to Him, knowing that “He has given us new birth into a living hope through the resurrection of Jesus Christ, and an inheritance that can never spoil or fade.” This will “result in praise, glory and honour when Jesus Christ is revealed” (1 Peter 1:3-9). When is he revealed? At his second coming, when he brings judgment.

Your question did not initially ask for any premillennial view / interpretation of 2 Thessalonians 1:9-10. It simply stated that when Christ's second coming is placed within the Day of Judgment, that view runs contrary to premillennialism.

Your link gives a premillennial view explaining their view, so we already have the answer to that. Well, all I can do is answer the question as it appears - you want to know if 2 Thessalonians 1:9-10 contradicts premillennialism. Here is an answer you might like to consider.

It is from verse 7 that the answer starts, first speaking of

"...when the Lord Jesus Christ shall be revealed from heaven with his mighty angels, in flaming fire taking vengeance on them that know not God, and that obey not the gospel of our Lord Jesus Christ."

We know from other texts, especially in Revelation, that it is Christ's sudden appearing, with the mighty angels and those who belong to him, that triggers the start of the Day of Resurrection and Judgment. Read, for instance, chapter 19 verses 11-21.

This means that both the apostles Paul and John were agreed that Christ's unexpected second coming is right at the start of the Day of Resurrection and Judgment. When he appears, it will be too late for any of the ungodly to repent; their judgment is then fixed, hence the nations and peoples of the earth will wail and mourn (Rev. 1:7) whilst those who belong to him will rejoice, being with him, 'on his side' at this awesome day.

EDIT in view of question now being edited: In conclusion, the points above highlight some of the inherent contradictions between premillennialism and 2 Thessalonians 1:9-10 which shows Christ's sudden appearing to trigger the Day [singlular] of Resurrection and Judgment, the Judgment being carried out after all have been given resurrection bodies in which to stand before the Throne of Judgment. No doubt premillennialists have a way of rationalising this, to their own satisfaction, but the text in question presents a total obstacle when it is left to speak, without attempts at rationalising it.