Why was Josiah's younger son made king initially?

2 Chronicles 35:20

After all this, when Josiah had set the temple in order, Necho king of Egypt went up to fight at Carchemish on the Euphrates, and Josiah marched out to meet him in battle.

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23 Archers shot King Josiah, and he told his officers, “Take me away; I am badly wounded.” 24 So they took him out of his chariot, put him in his other chariot and brought him to Jerusalem, where he died. He was buried in the tombs of his ancestors, and all Judah and Jerusalem mourned for him.

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Josiah appointed no clear successor. The people liked the younger Jehoahaz.

2 Chronicles 36:1 And the people of the land took Jehoahaz son of Josiah and made him king in Jerusalem in place of his father.

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But Necho didn't like Jehoahaz and replaced him with his older brother.

2 Jehoahaz a was twenty-three years old when he became king, and he reigned in Jerusalem three months. 3The king of Egypt dethroned him in Jerusalem and imposed on Judah a levy of a hundred talents b of silver and a talent of gold. 4The king of Egypt made Eliakim, a brother of Jehoahaz, king over Judah and Jerusalem and changed Eliakim’s name to Jehoiakim. But Necho took Eliakim’s brother Jehoahaz and carried him off to Egypt.

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Why was Josiah's younger son made king initially?

Jehoahaz was picked by the people, probably for political reasons. Jehoahaz stood with the people against Necho.

What would have been "normal" for succession?

The oldest son was expected to succeed, but it was not guaranteed.

After the death of his elder brothers Amnon and Absalom, Adonijah considered himself the heir apparent to the throne.

1 Kings 2:15 "As you know," he [Adonijah] said, "the kingdom was mine. All Israel looked to me as their king. But things changed, and the kingdom has gone to my brother; for it has come to him from the LORD.

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Was there any particular law on that point?

Kind of, but not exactly. It is the law of primogeniture.

Deuteronomy 21:15 If a man has two wives, and he loves one but not the other, and both bear him sons but the firstborn is the son of the wife he does not love, 16when he wills his property to his sons, he must not give the rights of the firstborn to the son of the wife he loves in preference to his actual firstborn, the son of the wife he does not love. 17 He must acknowledge the son of his unloved wife as the firstborn by giving him a double share of all he has. That son is the first sign of his father’s strength. The right of the firstborn belongs to him.

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