“For three years Aram and Israel continued without war. But in the third year King Jehoshaphat of Judah came down to the king of Israel. The king of Israel said to his servants. ‘Do you know that Ramoth-gilead belongs to us? Yet we are doing nothing to take it out of the hand of the king of Aram. He said to King Jehoshaphat. ‘Will you go with me to battle at Ramoth-gilead?’ King Jehoshaphat replied to the king of Israel. ‘I am as you are. My people are your people. My horses are your horses.’”
There was some kind of truce after the great defeat at Aphek in chapter 20. King Jehoshaphat of Judah came to King Ahab in some kind of reconciliation. King Ahab was consulting with his advisors or servants on whether to take back Ramon-gilead which was on the east side of the Jordan River. He asked King Jehoshaphat to come with him. His response was in complete agreement as he said that he, and his people and their horses were at his disposal.