John Gill 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)

When the king of Babylon's army fought against Jerusalem Had laid close siege to it, and still continued it:

and against all the cities of Judah that were left ; unconquered by him; when he invaded the land, he fought against, and took, and ravished all the cities that lay in his way; and it seems there were none that stood out against him but Jerusalem, now besieged by him, and two others, next mentioned:

against Lachish, and against Azekah; for these defenced cities remained of the cities of Judah ; two cities that had been fortified by Rehoboam, (2 Chronicles 11:9) ; and were the only ones besides Jerusalem, which as yet had not fallen into the hands of the king of Babylon.