דברי הימים ב כח:כא פירוש: רש״י

but he was of no help to him Even though we find in II Kings (ibid. verse 9): “and the king of Assyria went up to Damascus and seized it and exiled it[s inhabitants] to Kir, and slew Rezin,” he was not much of an aid to him to strengthen him and to restore the cities that the Philistines had taken from him.