Jehu’s movement to the throne began when God told Elijah to anoint him as king over Israel.  The command was actually carried out by Elijah’s successor Elisha.  Jehu was to overthrow King Ahab and Queen Jezebel, who had promoted the worship of foreign deities and persecuted God’s prophets.  Ahab was killed in battle before Jehu was anointed, but Jehu did bring about the death of Jezebel and Ahab’s descendents.