A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
2 Chronicles 13:2
ch2 13:2ABIJAH, SUCCEEDING, MAKES WAR AGAINST JEROBOAM, AND OVERCOMES HIM. (2Ch. 13:1-20)
His mother's name also was Michaiah, the daughter of Uriel--the same as Maachah (see on Kg1 15:2). She was "the daughter," that is, granddaughter of Absalom (Kg1 15:2; compare 2Sa. 14:1-33), mother of Abijah, "mother," that is, grandmother (Kg1 15:10, Margin) of Asa.
of Gibeah--probably implies that Uriel was connected with the house of Saul.
there was war between Abijah and Jeroboam--The occasion of this war is not recorded (see Kg1 15:6-7), but it may be inferred from the tenor of Abijah's address that it arose from his youthful ambition to recover the full hereditary dominion of his ancestors. No prophet now forbade a war with Israel (Ch2 11:23) for Jeroboam had forfeited all claim to protection.
2 Chronicles 13:3
ch2 13:3Abijah set the battle in array--that is, took the field and opened the campaign.
with . . . four hundred thousand chosen men . . . Jeroboam with eight hundred thousand--These are, doubtless, large numbers, considering the smallness of the two kingdoms. It must be borne in mind, however, that Oriental armies are mere mobs--vast numbers accompanying the camp in hope of plunder, so that the gross numbers described as going upon an Asiatic expedition are often far from denoting the exact number of fighting men. But in accounting for the large number of soldiers enlisted in the respective armies of Abijah and Jeroboam, there is no need of resorting to this mode of explanation; for we know by the census of David the immense number of the population that was capable of bearing arms (Ch1 21:5; compare Ch2 14:8; Ch2 17:14).
2 Chronicles 13:4
ch2 13:4Abijah stood up upon Mount Zemaraim--He had entered the enemy's territory and was encamped on an eminence near Beth-el (Jos 18:22). Jeroboam's army lay at the foot of the hill, and as a pitched battle was expected, Abijah, according to the singular usage of ancient times, harangued the enemy. The speakers in such circumstances, while always extolling their own merits, poured out torrents of invective and virulent abuse upon the adversary. So did Abijah. He dwelt on the divine right of the house of David to the throne; and sinking all reference to the heaven-condemned offenses of Solomon and the divine appointment of Jeroboam, as well as the divine sanction of the separation, he upbraided Jeroboam as a usurper, and his subjects as rebels, who took advantage of the youth and inexperience of Rehoboam. Then contrasting the religious state of the two kingdoms, he drew a black picture of the impious innovations and gross idolatry introduced by Jeroboam, with his expulsion and impoverishment (Ch2 11:14) of the Levites. He dwelt with reasonable pride on the pure and regular observance of the ancient institutions of Moses in his own dominion [Ch2 13:11] and concluded with this emphatic appeal: "O children of Israel, fight ye not against Jehovah, the God of your fathers, for ye shall not prosper."
2 Chronicles 13:13
ch2 13:13But Jeroboam caused an ambushment to come about behind them--The oration of Abijah, however animating an effect it might have produced on his own troops, was unheeded by the party to whom it was addressed; for while he was wasting time in useless words, Jeroboam had ordered a detachment of his men to move quietly round the base of the hill, so that when Abijah stopped speaking, he and his followers found themselves surprised in the rear, while the main body of the Israelitish forces remained in front. A panic might have ensued, had not the leaders "cried unto the Lord," and the priests "sounded with the trumpets"--the pledge of victory (Num 10:9; Num 31:6). Reassured by the well-known signal, the men of Judah responded with a war shout, which, echoed by the whole army, was followed by an impetuous rush against the foe. The shock was resistless. The ranks of the Israelites were broken, for "God smote Jeroboam and all Israel." They took to flight, and the merciless slaughter that ensued can be accounted for only by tracing it to the rancorous passions enkindled by a civil war.
2 Chronicles 13:19
ch2 13:19Abijah pursued after Jeroboam, and took cities from him--This sanguinary action widened the breach between the people of the two kingdoms. Abijah abandoned his original design of attempting the subjugation of the ten tribes, contenting himself with the recovery of a few border towns, which, though lying within Judah or Benjamin, had been alienated to the new or northern kingdom. Among these was Beth-el, which, with its sacred associations, he might be strongly desirous to wrest from profanation.
2 Chronicles 13:20
ch2 13:20Neither did Jeroboam recover strength again in the days of Abijah--The disastrous action at Zemaraim, which caused the loss of the flower and chivalry of his army, broke his spirits and crippled his power.
the Lord struck him, and he died--that is, Jeroboam. He lived, indeed, two years after the death of Abijah (Kg1 14:20; Kg1 15:9). But he had been threatened with great calamities upon himself and his house, and it is apparently to the execution of these threatenings, which issued in his death, that an anticipatory reference is here made.