Here's what happened.
There was a war between king Abijah of Judah and Jeroboam of Israel. Abijah had 400,000 "chosen men" and Jeroboam had 800,000 "mighty men of valor."
Abijah set the battle in array with an army of valiant men of war, even four hundred thousand chosen men: Jeroboam also set the battle in array against him with eight hundred thousand chosen men, being mighty men of valour. 2 Chronicles 13.3Abijah stood on a hilltop and delivered a long, boring speech to all 1.2 million men. (He had a very loud voice.)
Abijah stood up upon mount Zemaraim, which is in mount Ephraim, and said, Hear me, thou Jeroboam, and all Israel; Ought ye not to know that the LORD God of Israel gave the kingdom over Israel to David for ever, even to him and to his sons by a covenant of salt?
And, behold, God himself is with us for our captain, and his priests with sounding trumpets to cry alarm against you. O children of Israel, fight ye not against the LORD God of your fathers; for ye shall not prosper. 13.4-12
When Abijah was done with his speech, Jeroboam's mighty men attacked from behind.
But Jeroboam caused an ambushment to come about behind them: so they were before Judah, and the ambushment was behind them. 13.13But then the chosen men of Judah cried to God, blew some trumpets, and shouted all at once.
When Judah looked back, behold, the battle was before and behind: and they cried unto the LORD, and the priests sounded with the trumpets. Then the men of Judah gave a shout. 13.14-15aAnd then God got involved, killing 500,000 Israelite soldiers.
As the men of Judah shouted … God smote Jeroboam and all Israel before Abijah and Judah. And the children of Israel fled before Judah: and God delivered them into their hand. And Abijah and his people slew them with a great slaughter: so there fell down slain of Israel five hundred thousand chosen men. 13.15b-17But it was all for a good cause. I'm just not sure what the cause was.
God's next killing: Jeroboam