At a moment when Judah seems outnumbered and on the verge of total destruction by the Assyrians, God intervenes. The Assyrian King Sennacherib has practically just finished boasting about how he's defeated the gods of so many other nations, when God steps in and sends the Angel of Destruction down to deal with the Assyrian army. The Angel kills 180,000 Assyrian troops and Sennacherib is killed by a conspiracy when he returns to Assyria.
This is yet another symbol of how God's power trumps human power, like the sequence where Elisha's servant sees that the divine army is protecting them. But unlike with that particular incident, God uses extreme violence to make his point in this case. God has called Sennacherib to invade Judah—and he can destroy him too.