Notes on the Bible, by Albert Barnes, [1834], at sacred-texts.com
The history of Abijah's reign is here related far more fully than in Kings (marginal reference), especially as regards his war with Jeroboam.
See the Kg1 15:2 note.
It has been proposed to change the numbers, here and in Ch2 13:17, into 40,000, 80,000, and 50,000 respectively - partly because these smaller numbers are found in many early editions of the Vulgate, but mainly because the larger ones are thought to be incredible. The numbers accord well, however, with the census of the people taken in the reign of David Ch1 21:5, joined to the fact which the writer has related Ch2 11:13-17, of a considerable subsequent emigration from the northern kingdom into the southern one. The total adult male population at the time of the census was 1,570, 000. The total of the fighting men now is 1,200, 000. This would allow for the aged and infirm 370, 000, or nearly a fourth of the whole. And in Ch2 13:17, our author may be understood to mean that this was the entire Israelite loss in the course of the war, which probably continued through the whole reign of Abijah.
Seven rams - "A bullock and two rams" was the offering which God had required at the original consecration of the sons of Aaron Exo 29:1; Lev 8:2. Jeroboam, for reasons of his own, enlarged the sacrifice, and required it at the consecration of every priest.
Slain - The word means strictly "pierced," and will include both the killed and the wounded. It is translated "wounded" in Lam 2:12.
Brought under - "Humbled" or "defeated," not reduced to subjection.
Jeshanah is probably identical with the "Isanas" of Josephus, where a battle took place in the war between Antigonus and Herod; but its situation cannot be fixed. For Ephrain, see Jos 18:23 note.
Jeroboam's death was a judgment upon him for his sins. Chronologically speaking, his death is here out of place, for he outlived Abijah at least two years (compare the marginal reference and Kg1 15:9); but the writer, not intending to recur to his history, is naturally led to carry it on to its termination.