The Babylonians had already conquered Judah a decade prior to the more destructive conquest which is described in this chapter. At that time Jehoachin (or Jeconiah, or Coniah) had been the king of Jerusalem. This time Zedekiah, Jehoachin's uncle, was king. This means that Josiah had 3 different sons who reigned as kings in Jerusalem: Jehoahaz, Jehoakim and finally Zedekiah. During the days of Zedekiah God sent word to him through Jeremiah that he was supposed to submit to Nebuchadnezzar (Jeremiah 21:9)... but he refused.
When Nebuchadnezzar was finished putting down the insurrection which had been led by Zedekiah (II Kings 24:20), he killed Zedekiah's sons while forcing Zedekiah to watch it happen... then he blinded Zedekiah and took him as his prisoner. After that, Nebuchadnezzar's men completely destroyed both the temple of Solomon and the walls of Jerusalem. The fall of David's entire kingdom was complete.
Nebuchadnezzar did leave some poor Jewish peasants in the land to do some farming, and he promoted one Gedaliah to a new position in Jerusalem. Gedaliah was declared to be the governor. Well, he was governor until he was murdered by some of the people over whom he was supposed to be ruling. The eventual exaltation of Jehoachin in Babylon demonstrates that God's plan at this juncture in Hebrew history was for them to submit to the Babylonians (Jeremiah 29:7).