2 Kings 13:1

Jehoahaz. The record of Jehu’s death and of Jehoahaz’ accession to the throne is found in 2 Kings 10:35. But the record of Jehoahaz’ reign does not appear till this place, because of the fact that reigns are generally arranged in the books of Kings to accord with the order of accession of the rulers to the throne.

Only a few days, presumably, after Jehu began to reign in Israel, Athaliah took the throne of Judah. Then in the 7th year of Jehu, the child Joash began his long rule of 40 years in Judah. That is evidently why these three reigns appear in Kings in the above order: Jehu, Athaliah, Joash. Then Jehoahaz of Israel and his successor Jehoash, both of whom came to the throne during the lifetime of Joash of Judah, come next in the record. After this the narrative goes back again to Judah to record the reign of Amaziah, who succeeded Joash.