John Calvin 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)

And Rabshakeh having returned. The sacred historian now explains how Rabshakeh, without accomplishing anything, returned to his king, though not to the same place where he had left him. For Rabshakeh understood that the king of Assyria had lifted the siege of Lachish and had departed for Egypt to attack Libnah. Some think that this city is Pelusium, while others prefer to assign it to Judea.

It is indeed probable that, due to a report he received about the approach of enemies, the king of Assyria moved his camp towards Egypt, so that by meeting them he might prevent them from advancing. Though God restrained the tyrant’s violence with a new war to give some relief to the Jews, yet God did not wish to conquer the tyrant by human hands. His intention was only to show openly—and, as it were, display on a stage—the tyrant’s unconquerable pride. For even when in great danger, the tyrant did not cease to vomit out the same blasphemies, as we will soon see.