This marks the exact time, that it was early in the campaign, while the outlying fortresses still occupied the attention of Nebuchadnezzars army. Lachish and Azekah were strong cities in the plain toward Egypt and must be taken before the Chaldseans could march upon Jerusalem: otherwise the Egyptians might collect there and fall upon them.
Related Commentaries of Jeremiah 34:7
Jeremiah 34:7
7 When the king of Babylon's army fought against Jerusalem, and against all the cities of Judah that were left, against Lachish, and against Azekah: for these defenced cities remained of the cities of Judah.