2 Kings 19:34

For I will defend this city, to save it, for mine own sake, and for my servant David’s sake.

The promise was given to Hezekiah after the army of occupation had laid waste the land of Judah, together with miraculous provision for the needs of the people. God Himself would defend Jerusalem to save it, for His own sake and for His servant David's sake. This meant that Sennacherib king of Assyria would not come into the city, nor shoot an arrow there, nor come before it with shield, nor cast a bank against it, but by the way that he came, by the same he would return.

As a sign, Judah would eat in that first year what grows of itself, and in the second year what springs from the same, and then in the third year sow and reap and plant vineyards and eat the fruit thereof. The remnant that is escaped of the house of Judah would yet again take root downward and bear fruit upward. A remnant would go forth out of Jerusalem and out of Mount Zion, because the zeal of the Lord of hosts would do this.