The landmark event of the Assyrian Invasion was the very instance that the prophetic truth which had been consistently proclaimed by Isaiah the prophet (and others--which has been long ignored and dismissed), became an enormously looming reality in life that the whole nation had to confront. 

Although the formal engine of their religious (church) schedule of activity had been actively in motion; they had routinely ignored the Word of God declared by His true messengers and continued to follow the false counsel of their own whims. 

However, they could not ignore the witness of divine truth any longer. 
By the permissive will of God, the chastening rod of the Assyrian host had been unleashed against His people, and they were forced to face the painful reality of that fact.