The Lord shall bring upon thee, etc. The transition from promises to threatenings is abrupt, and calculated to impress any one who was to any extent impressible. But Ahaz seems not to have had "ears to hear."
From the day that Ephraim departed from Judah; i.e. from the time of the revolt under Jeroboam (1 Kings 12:16-24)—an evil day, which rankled in the mind of all true Judaeans. Even the King of Assyria.
The construction is awkward, since "the King of Assyria' cannot well stand in apposition with "days." Hence many take the words for a gloss that has been accidentally intruded into the text (Lowth, Gesenius, Hitzig, Knobel, Cheyne).
Others, however, see in the grammatical anomaly a grace of composition.