"Tirhakah" is the name of a man. He had mobilized his army so that they were ready to fight. The full meaning of this statement can be made clear. Alternate translation: "Tirhakah king of Cush and Egypt had mobilized his army" (See: [[rc://en/ta/man/translate/translate-names]] and [[rc://en/ta/man/translate/figs-explicit]])
The word "him" represents Sennacherib. Here Sennacherib represents his army. Alternate translation: "to fight against the army of Sennacherib" (See: [[rc://en/ta/man/translate/figs-metonymy]])