Albert Barnes 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)

And he heard say - The report or rumor referred to in Isaiah 37:7. In what way he heard this is not indicated. It is probable that the preparations Tirhakah had made were well known to the surrounding regions and that he was already on his march against Sennacherib.

Tirhakah - This king—whom Eusebius and most ancient writers call Ταρακὸς Tarakos—was a celebrated conqueror and had subdued Egypt for himself. He reigned over Egypt for eighteen years.

When Sennacherib marched into Egypt, Sevechus or Sethon was on the throne. After Sennacherib laid siege to Pelusium, Tirhakah came to the city's aid. In consequence of his aid, Sennacherib was compelled to raise the siege, returned to Palestine, and laid siege to Lachish.

Tirhakah succeeded Sevechus in Egypt and was the third and last of the Ethiopian kings who reigned over that country. He probably took advantage of the distracted state that followed the death of Sevechus and secured the crown for himself. This, however, was after the death of Sennacherib.

The capital which he occupied was Thebes (see Prideaux’s “Connection,” vol. i. pp. 141, 145, 149. Ed. 1815). As he was celebrated as a conqueror and had driven Sennacherib from Pelusium and from Egypt, we may see the cause of Sennacherib's alarm when it was rumoured that he was about to follow him into Palestine and to make war on him there.

He is come forth - He has made preparations and is on his way.

He sent messengers ... - With letters or dispatches (Isaiah 37:14). Hezekiah was probably ignorant of Tirhakah's approach; or, at all events, Sennacherib would suppose that he was ignorant of it. And since Sennacherib knew that there would be no hope of Hezekiah yielding if he knew that Tirhakah was approaching to make war on him, Sennacherib seems to have resolved to anticipate this intelligence. His further aim was to see if it were possible to induce him to surrender. He, therefore, sent substantially the same message as before and summoned him to capitulate.