Albert Barnes Commentary Isaiah 37:8

"So Rabshakeh returned, and found the king of Assyria warring against Libnah; for he had heard that he was departed from Lachish." — Isaiah 37:8 (ASV)

So Rabshakeh returned – Returned from Jerusalem to the camp of his master. He had received no answer to his insulting message (Isaiah 36:21); he saw there was no prospect that the city would surrender, and he therefore returned again to the camp.

And found the king of Assyria warring against Libnah – He had departed from Lachish. Why he had done this is unknown. It is possible that he had taken it, though this is not recorded anywhere in history. Or it is possible that he had found it impracticable to subdue it as speedily as he had desired, and had withdrawn from it for the purpose of subduing other places that would offer a more feeble resistance.

Libnah was a city in the south of Judah (Joshua 15:42), given to the priests, and declared a city of refuge (1 Chronicles 6:54), (1 Chronicles 6:57). Eusebius and Jerome say it was in the district of Eleutheropolis (Calmet). It was about ten miles to the northwest of Lachish. This city was taken by Joshua, and all its inhabitants were put to the sword. After taking this.

Joshua next assaulted and took Lachish (Joshua 10:29–32).