Complex sentence: Amaziah began to worship some of the idols he took from the Edomites, which the author of "Chronicles" believes led to his ruin and his defeat by Jehoash, king of Israel whom he had challenged to battle.
Split into simpler sentences: Amaziah began to worship some of the idols he took from the Edomites. He was defeated by Jehoash, king of Israel whom he had challenged to battle.