Albert Barnes Commentary Jeremiah 34:7

"when the king of Babylon`s army was fighting against Jerusalem, and against all the cities of Judah that were left, against Lachish and against Azekah; for these [alone] remained of the cities of Judah [as] fortified cities." — Jeremiah 34:7 (ASV)

This marks the exact time: it was early in the campaign, while the outlying fortresses still occupied the attention of Nebuchadnezzar's army. Lachish and Azekah were strong cities in the plain toward Egypt and had to be taken before the Chaldeans could march upon Jerusalem; otherwise, the Egyptians might gather there and attack them.