Charles Ellicott Commentary Isaiah 37:9

"And he heard say concerning Tirhakah king of Ethiopia, He is come out to fight against thee. And when he heard it, he sent messengers to Hezekiah, saying," — Isaiah 37:9 (ASV)

Tirhakah.—He was the third king of the twenty-fifth, or Ethiopian, dynasty. So, or Sabaco (with whom Hoshea, the last king of Israel, allied himself, 2 Kings 17:4), was the first of this dynasty. He is described in Assurbanipal’s inscriptions (Records of the Past, i. 60) as king of Mizr and Cush—i.e., Egypt and Ethiopia. The policy of Hezekiah’s counsellors had led them to seek his alliance, as in Isaiah 30:31. Now, however, the Egyptian army was at least mobilised. “Rahab” was no longer “sitting still” (Isaiah 30:7).

When he heard it.—The message was, in substance, a repetition of its predecessors, more defiant, perhaps, as if in answer to the threatened attack of Tirhakah’s armies, which Sennacherib could scarcely fail to connect with Hezekiah’s confident hope of deliverance.