For I will defend this city to save it for mine own sake, and for my servant David's sake.
I will defend this city. Notwithstanding Hezekiah's measures of defense (2 Chronicles 32:3-5), Yahweh was its true defender.
For mine own sake - since Yahweh's name was blasphemed by Sennacherib (Isaiah 37:23).
For my servant David's sake - on account of His promise to David (Psalms 132:17-18), and to Messiah, the heir of David's throne (Isaiah 9:7; Isaiah 11:1).