The Rabshakeh returned, and found the king of Assyria fighting against Libnah, for he heard that the king had left Lachish.
Rabshakeh, having delivered his message and received no answer (whether he took this silence for a consent or a slight does not appear), left his army before Jerusalem, under the command of the other generals, and went himself to attend the king his master for further orders. He found him besieging Libnah, a city that had revolted from Judah, Kg2 8:22.
Commenting on 2 Kings 19:8-19
SENNACHERIB SENDS A BLASPHEMOUS LETTER TO HEZEKIAH. (Kg2 19:8-13) So Rab-shakeh . . . found the king of Assyria warring against Libnah--Whether Lachish had fallen or not, is not said. But Sennacherib had transferred his battering-rams against the apparently neighboring fortress of Libnah (Jos 10:29; compare Jos 10:31; Jos 15:42), where the chief-cup-bearer reported the execution of his mission.
Libnah - Lachish - These two places were not very distant from each other; they were in the mountains of Judah, southward of Jerusalem.