Isaiah 7:6
Let us go up against Judah, and vex it, and let us make a breach therein for us, and set a king in the midst of it, even the son of Tabeal: Let us go up against Judah, and vex it, and let us make a breach therein for us, and set a king in the midst of it, even the son of Tabeal:
The verse records the joint hostile plan of Syria and Ephraim, that is Israel, against Judah. The plan was to go up against Judah, to invade it, and to make a breach therein by breaking through its defenses. The plan also aimed to vex Judah by throwing it into turmoil. The invaders intended to set the son of Tabeal as king in the midst of Judah in place of its king.
The threat was real. It left Ahaz and his people afraid. God declared through Isaiah that the plot would not succeed, so the alliance and its intended replacement of Judah's king would come to nothing.