After Rehoboam dies, his son Abijah takes over as planned.
Sadly, Abijah is still at war with Jeroboam in the north.
One day the two sides are getting ready to fight, but the Northern Kingdom has twice the soldiers that the Southern Kingdom does.
So Abijah stands on the slope of a mountain to deliver a long warning speech to Jeroboam. A sermon on the mount, if you will. (So that's where Jesus got the idea…)
Abijah tells Jeroboam and his followers that they have rebelled against God. After all, God appointed David and his sons to be the kings of Israel. Not Jeroboam and sons.
The king also accuses the northerners of worshipping other gods besides Yahweh and not only that, worshipping God in the wrong way.
The northerners can't make sacrifices in the Temple in Jerusalem because that's in the southern territory. Clearly, that's the only acceptable place to worship God.
The priests and Levites have all sworn allegiance to the south, and any priests not descended from Aaron (the high priest way back in Exodus) are not legit in the eyes of God. So basically, it's all a spiritual mess up in the north.
The Southern Kingdom, on the other hand, hasn't abandoned God. They've kept on doing all the things that God ordered them to do way back in Exodus. And that, northern scum, is why the south will win today. Game, set, match.
Nice speech. But while Abijah is talking, Jeroboam sends some guys to ambush him.
The people of Judah see they're surrounded, outmatched, and outnumbered. So they do the only thing they can—they ask God for help.
God comes to their aid and defeats the northerners. Abijah and the southern army are able to slaughter 500,000 men and take back some cities from Jeroboam.
God eventually kills Jeroboam.
In the meantime, Abijah gets stronger and stronger, which is a good thing because he has 14 wives and 38 children.