מדרש על דברי הימים ב 28:8

(Devarim 20:3) "And he shall say to them "Hear, O Israel … against your foes": and not against your brothers: not Yehudah against Shimon and not Shimon against Yehudah, in which instance if you fell into their hands they would pity you, (viz. II Chronicles 28:8-15) — It is against your foes that you are going, who, if you fall into their hands, will not pity you!