Now there is a switch in tone. Prior to this, Jeremiah and the oracles of Yahweh had said to obey the Babylonian king. Now there are a series of oracles against the Babylonians or the Chaldeans. Eventually Babylon would fall in 539 BCE, long after the death of Jeremiah (582 BCE) and King Nebuchadnezzar (562 BCE). In the Septuagint Greek translation, this is chapter 27, not chapter 50 as here.
Johanan, the son of Kareah, was one of the open field leaders against Babylon. He and the other rebel leaders came to Mizpah to see what was going on there. When he got there, he told Governor Gedaliah that the king of the Ammonites, King Baalis, had sent Ishmael, the son of Nethaniah, also another Judean country leader, to kill Gedaliah. They have actually found a seal from this King Baalis of Ammon dated around 600 BCE. However, Gedaliah did not believe him, since Ishmael was also a member of the Judean royal household.