So Jehu got up and went into the house, where the young prophet poured the oil on his head and declared, “This is what the LORD, the God of Israel, says: ‘I anoint you king over the LORD’s people Israel.
We have here the anointing of Jehu to be king, who was, at this time, a commander (probably commander-in-chief) of the forces employed at Ramoth-Gilead, Kg2 9:14. There he was fighting for the king his master, but received orders from a higher king to fight against him.
Commenting on 2 Kings 9:1-10
And thou shalt smite the house of Ahab thy master,.... For Jehu was one of Ahab's captains, as he was now one of his son's, as appears from Kg2 9:25, but this was to be no objection with him to the destroying of his house, as being an act of high treason, since he had an order for it from the King of kings, and...
And he arose, and went into the house; and he poured the oil on his head, and said unto him, Thus saith the LORD God of Israel, I have anointed thee king over the people of the LORD, [even] over Israel. (b) This anointing was for kings, priests and prophets which were all figures of Messiah, in whom these three offices were accomplished.