II Kings 11, Child King

The relatives of Ahaziah have been murdered by Jehu.

2 Kings 11: 1-3, Joash hidden

When Athaliah the mother of Ahaziah saw that her son was dead, she proceeded to destroy the whole royal family. But Jehosheba, the daughter of King Jehoram and sister of Ahaziah, took Joash son of Ahaziah and stole him away from among the royal princes, who were about to be murdered. She put him and his nurse in a bedroom to hide him from Athaliah; so he was not killed. He remained hidden with his nurse at the temple of the LORD for six years while Athaliah ruled the land.

Athaliah takes the throne and, like her brutal mother Jezebel before her, kills anyone who might threaten her reign.  These are family members she is killing, sons and nephews. But .Joash's mother (step-sister to Athaliah) hides her infant son with a nurse in the temple -- for six years.

2 Kings 11: 4-8, Bodyguards for Joash

In the seventh year Jehoiada sent for the commanders of units of a hundred, the Carites and the guards and had them brought to him at the temple of the LORD. He made a covenant with them and put them under oath at the temple of the LORD. Then he showed them the king's son. He commanded them, saying, "This is what you are to do: You who are in the three companies that are going on duty on the Sabbath--a third of you guarding the royal palace, a third at the Sur Gate, and a third at the gate behind the guard, who take turns guarding the temple-- and you who are in the other two companies that normally go off Sabbath duty are all to guard the temple for the king.

Station yourselves around the king, each man with his weapon in his hand. Anyone who approaches your ranks must be put to death. Stay close to the king wherever he goes."
 

The priest, Jehoiada, sets up a protective guard for Joash. (According to 2 Chronicles 22: 11-12, Jehoiada is the husband of Jehosheba and has been working with her for these six years to protect Joash.)  The protective guard is made up of commanders of the soldiers, yet presumably Jehoiada is aware that they can be trusted in this coup.

2 Kings 11: 9-12, Coronation

The commanders of units of a hundred did just as Jehoiada the priest ordered. Each one took his men--those who were going on duty on the Sabbath and those who were going off duty--and came to Jehoiada the priest. Then he gave the commanders the spears and shields that had belonged to King David and that were in the temple of the LORD. The guards, each with his weapon in his hand, stationed themselves around the king--near the altar and the temple, from the south side to the north side of the temple.

Jehoiada brought out the king's son and put the crown on him; he presented him with a copy of the covenant and proclaimed him king. They anointed him, and the people clapped their hands and shouted, "Long live the king!"

After making sure that Joash is protected, Jehoiada then brings him out to be crowned. This coronation appears to be on the Sabbath, when some temple activity is expected, and so all this happens without Athaliah's knowledge.

2 Kings 11: 13-16, "Treason!"

When Athaliah heard the noise made by the guards and the people, she went to the people at the temple of the LORD. She looked and there was the king, standing by the pillar, as the custom was. The officers and the trumpeters were beside the king, and all the people of the land were rejoicing and blowing trumpets. Then Athaliah tore her robes and called out, "Treason! Treason!"

Jehoiada the priest ordered the commanders of units of a hundred, who were in charge of the troops: "Bring her out between the ranks and put to the sword anyone who follows her." For the priest had said, "She must not be put to death in the temple of the LORD." So they seized her as she reached the place where the horses enter the palace grounds, and there she was put to death.

 

Athaliah hears the noise of the coronation of Joash and hurries to find out its reason. Angry at the site of a new king (a boy of seven) she shouts "Treason!" She apparently runs from the temple and is captured "where horses enter palace grounds" and there, outside the temple, she is killed. (The priest makes sure she is killed outside the temple; presumably her blood in the temple would be unholy.)

The overthrow of Athaliah and the rise of Joash is covered in more detail in 2 Chronicles 22-23.

2 Kings 11: 17-21, Joash installed on the throne

Jehoiada then made a covenant between the LORD and the king and people that they would be the LORD's people. He also made a covenant between the king and the people. All the people of the land went to the temple of Baal and tore it down. They smashed the altars and idols to pieces and killed Mattan the priest of Baal in front of the altars. Then Jehoiada the priest posted guards at the temple of the LORD. He took with him the commanders of hundreds, the Carites, the guards and all the people of the land, and together they brought the king down from the temple of the LORD and went into the palace, entering by way of the gate of the guards. 

The king then took his place on the royal throne, and all the people of the land rejoiced. And the city was quiet, because Athaliah had been slain with the sword at the palace.

Joash was seven years old when he began to reign.

Joash is installed on the throne and, says the text, then the people rejoice. The writer notes that the city is quiet after the death of Athaliah -- Athaliah's reign was a corrupt one, stressful and chaotic for the people of Jerusalem.

The installation of Joash involves two covenants, one between YHWH, the king and the people -- presumably a covenant following Mosaic Law -- and then a covenant between the people and the king. All of this is organized by the priest Jehoiada and is presumably an attempt to return the reign to the theocracy implied in the covenants of Moses and David.