מלכים ב, פרק י״ט, פסוק ל״ו

The sudden collapse of a powerful army leaves its leader with no choice but to abandon his campaign. Although Sennacherib himself survived and was not struck down in the battle, the destruction of his large army caused an immediate retreat. Without his troops, he was forced to turn back and return home [ביאור שטיינזלץ].

He made his way back to Nineveh, the capital city and the center of government for the Assyrian empire [רש״י, מצודת דוד]. His return was permanent. From that moment forward, the defeated king locked himself inside his city and never went out to fight another war [רד״ק].