Amaziah was twenty-five years old when he became king and reigned twenty-nine years in Jerusalem. His mother was Jehoaddin from Jerusalem.
He lived well before God, doing the right thing for the most part. But he wasn't wholeheartedly devoted to God.
When he had the affairs of the kingdom well in hand, he executed the palace guard who had assassinated his father the king.
But he didn't kill the sons of the assassins - he was mindful of what God commanded in The Revelation of Moses, that parents shouldn't be executed for their childrens' sins, nor children for their parents'. We each pay personally for our sins.
Amaziah organized Judah and sorted out Judah and Benjamin by families and by military units. Men twenty years and older had to register - they ended up with 300,000 judged capable of military service.
In addition he hired 100,000 soldiers from Israel in the north at a cost of about four and a half tons of silver.