Albert Barnes Commentary Isaiah 36:10

"And am I now come up without Jehovah against this land to destroy it? Jehovah said unto me, Go up against this land, and destroy it." — Isaiah 36:10 (ASV)

And am I now come up without the Lord - Am I come up without his permission or command? Rabshakeh here speaks in the name of his master, and he means to say that he had the express command of Yahweh to inflict punishment on the Jews.

It is possible that a rumor had been conveyed to Sennacherib of what Isaiah had said that God would bring the Assyrians upon the Jewish people to punish them for their sins, and that Rabshakeh now pleads that as his authority, in order to show them that resistance would be vain. Or it may be that he uses the name Yahweh here as synonymous with the name of God, and means to say that he had been divinely directed to come up in that expedition. All ancient warriors usually consulted the gods and endeavored by auguries to obtain the divine approbation of their plans of conquest, and Rabshakeh may mean simply to say that his master came now under divine sanction and direction.

Or, which is more probable, he made use of this as a mere pretense for the purpose of influencing the people who heard him, and to whom he said he was sent (Isaiah 36:12), in order to alienate their minds from Hezekiah and to induce them to surrender. He knew that it was one of the principles of the Jews, however little they regarded it in practice, to yield to his authority. Wicked people will be glad to plead divine authority for their purposes and plans when they can have the slightest pretense for it.