How did Amasias move from obeying a warning to defeat by Israel?

Amasias was twenty-five years old when he began to reign, and he reigned for twenty-nine years in Jerusalem; the name of his mother was Joadan of Jerusalem. He did that which was good in the sight of the Lord, though not with a perfect heart. When his kingdom was firmly established, he executed the servants who had slain his father the king, but he did not put their children to death, observing the prescription written in the Law of Moses: 'The fathers shall not be slain for the children, nor the children for their fathers, but every man shall die for his own sin.'

Preparing for war against the Edomites of Mount Seir, Amasias gathered the men of Juda and Benjamin, numbering them from twenty years old and upwards, and found three hundred thousand chosen men capable of going to battle and holding spear and shield. In addition, he hired one hundred thousand valiant soldiers from the kingdom of Israel for one hundred talents of silver. Before the expedition marched, a man of God arrived and warned him: 'O king, let not the army of Israel go out with thee, for the Lord is not with Israel, and all the children of Ephraim.' The prophet declared that if he relied on these troops, God would cause him to fall before the enemy, for God has power both to help and to put to flight. When Amasias expressed concern over what would become of the one hundred talents already given to the soldiers of Israel, the man of God answered that the Lord is rich enough to be able to give him much more than that. Amasias obeyed and separated the army that had come to him out of Ephraim, sending them home, which left the dismissed soldiers greatly enraged against Juda. Amasias then led his own people to the Valley of Saltpits, where they slew ten thousand of the children of Seir in combat and captured another ten thousand, executing the prisoners by casting them down headlong from the top of a steep rock. Meanwhile, the soldiers whom Amasias had sent back raided the cities of Juda from Samaria to Beth-horon, killing three thousand people and taking away extensive spoil.

Following his return from the slaughter of the Edomites, Amasias brought back the gods of the children of Seir, set them up to be his own gods, and adored them, burning incense before them. The Lord, being angry against Amasias, sent a prophet to rebuke him, asking why he adored gods that had failed to deliver their own people from his hand. The king answered haughtily, asking if the prophet had been appointed of the king's council, and commanded him to be quiet lest he be killed. The prophet forbore, but declared that he knew the Lord was minded to kill the king because he had done this evil and had refused prophetic counsel.

Taking ill counsel, Amasias sent messengers to Joas king of Israel, challenging him: 'Come, let us see one another.' Joas replied with a parable, telling how a thistle in Libanus sent to a cedar in Libanus asking for a marriage alliance for his son, but a wild beast passed by and trod down the thistle. Joas cautioned Amasias that his heart was lifted up because he had beaten Edom, urging him to remain at home rather than provoke misfortune that would bring about his downfall and the ruin of Juda. Amasias refused to listen, for it was the will of the Lord to deliver him into the hands of enemies because he had sought the gods of Edom. The two kings met at Bethsames in Juda, where the army of Juda was routed before Israel and fled to their dwellings. Joas captured Amasias, brought him to Jerusalem, demolished four hundred cubits of the city wall from the Ephraim Gate to the Corner Gate, took the gold, silver, and vessels found in the house of God and with Obededom, took the treasures of the king's house and the sons of the hostages, and returned to Samaria. Amasias lived for fifteen years after the death of Joas king of Israel. After Amasias departed from the Lord, a conspiracy was formed against him in Jerusalem; he fled to Lachis, but conspirators pursued him and slew him there, carrying his body back upon horses to be buried with his fathers in the city of David.