“When all Israel heard that Jeroboam had returned, they sent and called him to the assembly. They made him king over all Israel. There was none who followed the house of David, except the tribe of Judah alone.”
Jeroboam was declared king of Israel, just as Yahweh had wanted via the prophet Ahijah. Only the tribe of Judah did not follow all Israel. So now we have a king in Judah and a king of all Israel. Apparently Jeroboam who had worked for Solomon knew how badly the tribes of the north were being treated by Solomon.