John Gill Commentary 2 Chronicles 13

Now in the eighteenth year of King Jeroboam began Abijah to
reign over Judah.] (See Gill on 1 Kings 15:1).

He reigned three years in Jerusalem
(See Gill on 1 Kings 15:2),

his mother's name also was Michaiah the daughter of Uriel of Gibeah ;
see (2 Chronicles 11:20) , (See Gill on 1 Kings 15:2).

and there was war between Abijah and Jeroboam ;
and in this chapter is an account of a battle fought between them, not recorded in the book of Kings.

And Abijah set the battle in array, with an army of valiant man
of war, even four hundred thousand chosen men
Collected such an army of select men, led them into his enemy's country, and set them in order of battle:

and Jeroboam also set the battle in array against him, with eight
hundred thousand chosen men, being mighty men of valour ;
double the number of Abijah s army, he having ten tribes to collect out of, and Abijah but two.

And Abijah stood upon Mount Zemaraim, which is in Mount
Ephraim
Which might have its name from a city of Benjamin of this name, to which it was near, though within the borders of Ephraim, (Joshua 18:22) formerly inhabited by the Zemarites, from whence it might have its name, (Genesis 10:18) here Abijah stood, that he might be the better heard by the armies pitched in the valley; and very probably he desired a parley, and it was granted, otherwise he would not have been safe in the position in which he was;

and said, hear me, thou Jeroboam, and all Israel ;
as many as were now gathered together, and which were a great number.

Ought you not to know
They did know what he afterwards says, but he would have them consider and acknowledge it:

that the Lord God of Israel gave the kingdom over Israel to David for
ever ;
to him and his seed, particularly to the Messiah, that should spring from him; but whether Abijah had this in view is a question, see (2 Samuel 7:13 2 Samuel 7:16)

even to him and to his sons by a covenant of salt ?
that is, a perpetual one, which was inviolable, and never to be made void; called so, because salt preserves from corruption and putrefaction, and because made use of in sacrifices offered when covenants were made; the Targum is, "as salt waters, which never lose their saltness."