The Assyrian king launches a calculated psychological attack aimed at breaking the spirit of the people of Jerusalem. He builds his argument on the historical track record of the Assyrian empire, pointing out a harsh reality to the besieged citizens: every nation previously conquered by Assyria had prayed to their own gods and relied heavily on divine protection. Yet, not a single one of those nations was saved, either by their deities or by their armies.
Based on this unbroken streak of victories, the king mocks the people of Judah for placing their trust in God. He challenges them to explain why God would somehow succeed in saving them when every other deity in the region had utterly failed [ביאור שטיינזלץ]. When the king boasts about the nations that his ancestors completely wiped out and killed [ביאור שטיינזלץ], he is specifically referring to the military campaigns of the earlier Assyrian kings who preceded him, namely Pul and Tiglath-Pileser [רש״י].