The immediate danger posed by the kings of Aram and Israel may seem frightening, but a far more devastating crisis is on the horizon [ביאור שטיינזלץ, אברבנאל]. Despite clear warnings from the prophet, King Ahaz refused to trust God and instead reached out to the King of Assyria for protection. As a direct result, God will use that very same empire to punish the king and bring ruin to his land [מלבי״ם].
The impending disaster will target the king, his people, and his royal lineage. The initial wave of trouble will strike Ahaz himself when Tiglath-Pileser, the Assyrian king, attacks and besieges him [מצודת דוד, רד״ק]. A later blow will fall upon his son, King Hezekiah. During his reign, Sennacherib of Assyria will invade and conquer all the fortified cities of Judah [מצודת דוד, רד״ק, אברבנאל]. This second invasion will occur after Samaria and Damascus have already been sent into exile [אבן עזרא]. Notably, the warning is directed at the royal house rather than explicitly mentioning the king's son. This is a deliberate sign of respect for Hezekiah; because Ahaz was wicked and his son was righteous, Hezekiah is associated with the noble house of David rather than his immediate father.
The severity of this coming period is compared to the historic division of the kingdom during the days of Jeroboam. Ever since that monumental fracture, which sparked the beginning of Israel's hardships, there has not been a time of such intense suffering [רש״י, מצודת דוד, רד״ק]. This comparison highlights not only the physical destruction but also the overwhelming power of the enemy and the deep terror that will grip the nation [שד״ל]. Furthermore, just as the tribes of Ephraim broke away from Judah during that ancient split, the looming Assyrian invasion will result in the complete downfall of Ephraim [ביאור שטיינזלץ].
The specific identity of the attacker is stated clearly to remove any doubt about who will bring these unprecedented days of hardship [שד״ל, אברבנאל]. It is the King of Assyria who will unleash this massive destruction across the land, eventually leaving only Jerusalem trapped under a tight siege until God performs a miracle to save Hezekiah [רש״י, מצודת דוד, רד״ק].