Historical and military outcomes are often decided not on the battlefield, but in the mind. When faced with overwhelming threats, a shift in human psychology can change the course of empires. To calm King Hezekiah and remove his fear of the looming Assyrian army, God promises a hidden intervention directly within the enemy’s consciousness.
Instead of an immediate military defeat, God orchestrates a psychological miracle. The primary approach among commentators is that God instills a sudden panic or a sharp change of will within the Assyrian king [מצודת ציון, מלבי״ם, שטיינזלץ, שד״ל]. God alters the king's mindset, causing him to take a mere passing rumor with absolute seriousness [מלבי״ם]. This rumor involves news that the King of Cush is marching out to wage war against Assyria [רש״י, מצודת דוד]. Gripped by this sudden report, the Assyrian king decides to abandon his siege of Jerusalem and redirect his forces to fight this new threat [רש״י, מלבי״ם].
This indirect method raises a question about why God chooses to delay a direct strike on the Assyrian camp. One explanation is that the Assyrian king still has an unfulfilled historical purpose. He must first conquer Cush and Egypt to fulfill earlier prophecies. In doing so, he will gather vast amounts of wealth and loot, which are ultimately destined to fall into Hezekiah's hands [מלבי״ם].
Another question arises regarding the prophecy's silence on the famous, miraculous event where an angel destroys the Assyrian camp in a single night. At this specific moment, God reveals only what is strictly necessary to ease Hezekiah's immediate anxiety. Promising the overnight destruction of a massive army might have seemed entirely unbelievable to Hezekiah at the time. Furthermore, such a promise might have sparked moral questions about why an entire army should be wiped out solely because of the sins and arrogance of their king. As a result, the prophecy focuses strictly on the personal downfall of the Assyrian ruler [שד״ל].
The king's immediate departure from Jerusalem is not the end of the story. Eventually, he will return to Jerusalem, where his massive army will finally be destroyed. Only after this devastating loss will he retreat to his own country, entirely broken and defeated [רש״י, מצודת דוד, מלבי״ם]. Once back in his homeland, he will face his ultimate end, assassinated by the sword at the hands of his own sons. This highly personal punishment serves as a public proof that his arrogance and insults directed at God would never go unpunished [מלבי״ם, שד״ל].