Babylon Herself Will Eventually Be Subject To YHWH's Judgment (Jeremiah 25:12-14).
“And it will come about, when seventy years are accomplished, that I will punish the king of Babylon, and that nation, the word of YHWH, for their iniquity, and the land of the Chaldeans, and I will make it desolate for ever.”
But once the ‘seventy years', the period determined by God, was over, the then king of Babylon, together with the whole nation of Babylon would be punished because of their iniquity. And this was the sure word of YHWH. So like the Assyrians before them, having been the ‘rod of YHWH's anger' (Isaiah 10:5), they would be punished because they themselves were utterly sinful. And eventually the whole land of the Chaldeans would be made desolate into the distant future. It would nevermore attain its former glory. As with much prophecy the timing for the last part (‘I will make it desolate for ever') is vague (it will happen some time). It is the fact that is important.
That the mighty Babylonian empire would only last for around seventy years must have seemed inconceivable at the time to anyone who had no divine help in understanding the situation. The Assyrian empire had lasted far longer. But Jeremiah was to be proved correct.
“And I will bring on that land all my words which I have pronounced against it, (even all that is written in this book, which Jeremiah has prophesied against all the nations).”
The doom of Babylon had been already proclaimed by Isaiah long before (e.g. Isaiah 13-14), because of its arrogant pride. To Isaiah Babylon had symbolised the world in rebellion against God from the time of Babel onwards and he had prophesied its utter ruin. The note in brackets, referring to prophecies of Jeremiah made around this time (the fourth year of Jehoiakim; Jeremiah 45:1) and later recorded in his book (e.g. chapter 50), was probably added by his amanuensis. (LXX actually introduces Chapter s 46-51 around this point).
“For many nations and great kings will make bondmen of them, even of them, and I will recompense them according to their deeds, and according to the work of their hands.”
For Babylon also would be caught up in the tide of history and many great nations and great kings would arise and would bring Babylonia into bondage, commencing with Cyrus the Persian who would later be followed by Alexander the Great. Babylon too would be recompensed for their behaviour and doings, and also for their idolatry (compare Isaiah 47:11-15).