When Abijah dies, his son Asa takes over the Kingdom of Judah.
Asa keeps Judah on the right path. He gets rid of all references to foreign gods and encourages the people to follow God's law.
He also builds up Judah's defenses and army in various cities. Even though there aren't any wars for 10 years, this is a smart move because eventually the Ethiopians attack Judah.
Zerah the Ethiopian comes at them with a million soldiers. You read that right. Judah has about 300,000.
Totally outnumbered, Asa leads the army into battle and does pretty much the only thing he can do right then—he prays. God helps the strong and the weak. And boy, is Judah weak right now.
The Almighty hears the king's panicked cries and responds with a sweeping victory. Not only do they drive back the million-man army, Judah manages to kill every last one. No exaggeration whatsoever there.
The warriors in Judah are able to get all kinds of booty from the Ethiopians, so it's a pretty big win for them.