Historical note: Ahaz is the successor to Uzziah. As Isaiah notes in 2 Nephi 16:1, his call as a prophet occurs in the year of Uzziah's death, and therefore at the beginning of the reign of Ahaz. Isaiah gives the genealogy of Ahaz as the grandson of Uzziah. In the custom of the Southern kingdom, Ahaz had, for at least some time, served as a co-regent. This practice smoothed the political transition of kings. and made for a more consistent transfer of power in the southern kingdom than in the north, where regnal transition worked more on the Davidic model of prophetic anointing and popular acclaim. (Gottwald, p. 346-7.)
The northern kingdom's practice left the line of succession more open, and a comparison of the king lists for each kingdom confirms the greater turnover and change of dynasties in the northern kingdom.
The political situation in the beginning of this chapter of Isaiah highlights the division between the northern and southern kingdoms. As with much of the history of the Hebrews, tribal affiliations underlay much of the politics, and the separation of the two kingdoms was a renewal of the rivalry between Judah and the Ephraim-Manassah-Benjamin power centers that were united by David. (Gottwald, p. 242.) The tribal divisions becomes important in understanding Isaiah, particularly in this chapter, where the house of David becomes the lineage designation for the southern kingdom, and Ephraim becomes the tribal division that signifies the northern kingdom of Israel.
While both descended from the Davidic/Solomonic unified kingdom, by this point they have been separate for over 200 years, and following differing independent strategies of political alliance. At this point in time, the king of Israel (the northern kingdom) has allied with Syria.
The politics of the time were very complicated, with independent countries bolstering their intentions with alliances with other nations. In this particular instance, Syria has allied with Israel, and has intentions of conquering Judah by pressing against Jerusalem. Ahaz is not unaware of this Syro-Israelite alliance, and has made an alliance of his own with Assyria. While Ahaz is placing his confidence in this political alliance, Isaiah will tell him he should rather place his confidence in the Lord. Through the course of this prophecy to Ahaz, Isaiah will tell him that the consequences of the Judah-Assyria alliance will be only temporarily beneficial, and eventually disastrous.
"At the time the Immanuel prophecy was given (about 734 B.C.) the country of Judah was under threat of attack by Rezin, king of Syria, and Pekah, king of Israel. These kings had formed an alliance during the final part of the reign of Jotham, the predecessor of Ahaz, and had made war against Jerusalem but had not been able to prevail against it. (See 2 Kgs. 15:37; 16:5.) When Ahaz came to the throne in 735 B.C., the Syro-Ephraimite coalition made a renewed effort to take Jerusalem. The alliance had as its primary goal the unification of all the countries in the area into a solid anti-Assyrian block. When Ahaz refused to join, Rezin and Pekah decided to subjugate Judah and replace Ahaz with a leader more sympathetic to their anti-Assyrian policies." (Ludlow, p. 139.)