In trying to figure out what the great tribulation actually is, we need to think about what it means to be so 'great' as not to have occurred before, nor to ever occur again.
We are already given a hint, in that it is to be on 'Jerusalem,' meaning, 'a holy place,' which, in turn, is interpreted for us as 'this people' upon whom 'justice is to be meted out' according to 'all the things written' in prophetic Scripture. Lu 21:20-24
This means that it is the greatest tribulation or persecution of God's people ever, in both past and future, not the greatest calamity to occur to mankind at large.
How does the flood of Noah compare to the great tribulation?
How does the flood compare to the sword stroke?
Did a great crowd that no one was able to number survive Noah's flood?
Is the sword stroke on the beast so great that afterward there is no "all the earth" left to 'follow it with admiration? Re 13:3
Hence, both the great tribulation and the sword stroke - although they are two different events - are nothing compared to the calamity for mankind that was the flood of Noah, which wiped out almost all mankind, which can only mean that the great tribulation will be the greatest persecution of God's people - loosely defined - to ever have happened, dwarfing the 70 AD sacking of Jerusalem.
70 AD was quite unsettling that one can only imagine what a greater tribulations might be. Here's some notes from Josephus who bared witness to the accounts.
For then shall be great tribulation, such as was not since the beginning of the world to this time, no, nor ever shall be. Matthew 24:21
A preview of what's to come on a grander scale worldwide .
An excerpt from bible hub.
There shall be great tribulation - The word "tribulation" means calamity or "suffering." Luke 21:24 has specified in what this tribulation would consist: "They shall fall by the edge of the sword, and shall be led away captive into all nations, and Jerusalem shall be trodden down of the Gentiles, until the times of the Gentiles shall be fulfilled." That is, until the time allotted for the Gentiles "to do it" shall be fully accomplished, or as long as God is pleased to suffer them to do it.
The first thing mentioned by Luke is, that they should fall "by the edge of the sword" - that is, would be slain in war, as the sword was then principally used in war. This was most strikingly fulfilled. Josephus, in describing it, uses almost the very words of our Saviour. "All the calamities, says he, which had befallen any nation from the beginning of the world" were but small in comparison with those of the Jews. - Jewish Wars, b. i. preface, section 4.
He has given the following account of one part of the massacre when the city was taken: "And now, rushing into the city, they slew whomsoever they found, without distinction, and burned the houses and all the people who had fled into them; and when they entered for the sake of plunder, they found whole families of dead persons, and houses full of carcasses destroyed by famine, then they came out with their hands empty. And though they thus pitied the dead, they had not the same emotion for the living, but killed all they met, whereby they filled the lanes with dead bodies. "The whole city ran with blood," insomuch that many things which were burning were extinguished by the blood." - "Jewish Wars," b. 6 chapter 8, section 5; chapter 9, section 2, 3. He adds that in the siege of Jerusalem not fewer than "eleven hundred thousand" perished (Jewish Wars, b. 6 chapter 9, section 3) - a number almost half as great as are in the whole city of London. In the adjacent provinces no fewer than "two hundred and fifty thousand" are reckoned to have been slain; making in all whose deaths were ascertained the almost incredible number of "one million three hundred and fifty thousand" who were put to death.
These were not, indeed, all slain with the sword. Many were crucified. "Many hundreds," says Josephus ("Jewish Wars," b. v. chapter 11, section 1), "were first whipped, then tormented with various kinds of tortures, and finally crucified; the Roman soldiers nailing them (out of the wrath and hatred they bore to the Jews), one after one way and another after another, to crosses, "by way of jest," until at length the multitude became so great that room was lacking for crosses, and crosses for the bodies." So terribly was their imprecation fulfilled - "his blood be on us and on our children," Matthew 27:25. If it be asked how it was possible for so many people to be slain in a single city, it is to be remembered that the siege of Jerusalem commenced during the time of the Passover, when all the males of the Jews were required to be there, and when it is estimated that more than "three million" were usually assembled. See Josephus, Jewish Wars, b. 6 chapter 9, section 3, 4.
A horrible instance of the distress of Jerusalem is related by Josephus. The famine during the siege became so great that they ate what the most sordid animals refused to touch. A woman of distinguished rank, having been plundered by the soldiers, in hunger, rage, and despair, killed and roasted her own babe, and had eaten one half of it before the deed was discovered. - Jewish Wars, b. 6 chapter 3, section 3, 4. This cruel and dreadful act was also in fulfillment of prophecy, Deuteronomy 28:53, Deuteronomy 28:56-57.
Another thing added by Luke Luk 21:24, was, that "they should be led away captive into all nations." Josephus informs us that the captives taken during the whole war amounted to "ninety-seven thousand." The tall and handsome young men Titus reserved for triumph; of the rest, many were distributed through the Roman provinces to be destroyed by wild beasts in theaters; many were sent to the works in Egypt; many, especially those under seventeen years of age, were sold for slaves. - Jewish Wars, b. vi. chapter 9, section 2, 3.
