Jehu was punishing Ahab’s family. Jehu found the leaders of Judah and Ahaziah’s relatives that served Ahaziah. Jehu killed those leaders of Judah and Ahaziah’s relatives.
The Aramean army came with a small group of men, but the Lord let them defeat a very large army from Judah. The Lord did this because the people of Judah left the Lord God their ancestors {\cf2\super [282]} followed. So Joash was punished.