Albert Barnes Commentary 1 Kings 22:4

"And he said unto Jehoshaphat, Wilt thou go with me to battle to Ramoth-gilead? And Jehoshaphat said to the king of Israel, I am as thou art, my people as thy people, my horses as thy horses." — 1 Kings 22:4 (ASV)

Ahab, well aware of Syria's military strength and feeling he could no longer expect divine aid (1 Kings 20:42; 1 Kings 21:21), requested help from Jehoshaphat, whose military resources were very great (2 Chronicles 17:12–19). Jehoshaphat’s response was one of complete agreement, without any reservation (compare 2 Chronicles 18:3).

Jehoshaphat was later rebuked for consenting to help the ungodly (2 Chronicles 19:2). He probably acted not merely out of a desire to please, but from a belief that this action would advance the interests of his own kingdom, since the power of Syria was very menacing at this time.