Albert Barnes Commentary 2 Kings 20:6

"And I will add unto thy days fifteen years; and I will deliver thee and this city out of the hand of the king of Assyria; and I will defend this city for mine own sake, and for my servant David`s sake." — 2 Kings 20:6 (ASV)

The king of Assyria in 714 and 713 B.C. was Sargon (721–705 B.C.). If the biblical and Assyrian chronologies, which agree exactly on the year Samaria was taken (721 B.C.), can be relied upon, then the king of Assyria mentioned here must have been Sargon. It may be conjectured that he had taken offense at something in Hezekiah's conduct and threatened Jerusalem around this time . There is, however, no evidence of actual hostilities between Judea and Assyria during Sargon’s reign.