Judah, Hezekiah, and Isaiah 28-35

"The key issue in chapters 28-35 is whether Judah, and in particular its leaders, will rely on Egypt or on the LORD in the face of the growing threat posed by the ever-increasing power of Assyria.  Chapters 30 and 31, which stand more or less centrally within the unity, are wholly taken up with this issue, with 31:1 providing perhaps the most pointed and succinct statement of it:

Woe to those who go down to Egypt for help,

who rely on horses,

who trust in the multitude of their chariots...

but do not look to the Holy one of Israel,

or seek help from the LORD

...the invasion (of Judah by Sennacherib) was a punitive action taken by Sennacherib in response to a revolt led by Hezekiah.  He had refused to pay any further tribute to Assyria and had annexed all the Philistine cities as far south as Gaza, cities which, like Judah, had been part of the Assyrian empire." - Webb, p116

Webb notes this passage for support - 

2 Kings 18:7–8 (NKJV)

⁷ The Lord was with him; he prospered wherever he went. And he rebelled against the king of Assyria and did not serve him. ⁸ He subdued the Philistines, as far as Gaza and its territory, from watchtower to fortified city.