Now in the eighteenth year of King Jeroboam the son of Nebat, Abijam began to reign over Judah.
We have here a short account of the short reign of Abijam the son of Rehoboam king of Judah. He makes a better figure, 2 Chr. 13, where we have an account of his war with Jeroboam, the speech which he made before the armies engaged, and the wonderful victory he obtained by the help of God.
Commenting on 1 Kings 15:1-8
Now in the eighteenth year of King Jeroboam the son of Nebat reigned Abijam over Judah. That is, began to reign; and by this it appears that Rehoboam was in the eighteenth year of his reign when he died, for he and Jeroboam began their reign at the same time. Now in the eighteenth year of King Jeroboam the son of Nebat reigned Abijam over Judah.
And Rehoboam slept with his fathers, and was buried with his fathers in the city of David. And his mother's name [was] Naamah an Ammonitess. And Abijam his son reigned in his stead. (t) Whose idolatry Rehoboam her son followed.