Or HaChaim on Numbers 27:21/אור החיים על במדבר כ״ז:כ״א

1 על פיו יצאו, "at his command they will go out (to war), etc." This was a reference to Moses having said that a leader must be capable of commanding the people to follow him, i.e. ואשר יוציאם. All of this would occur after Eleazar had obtained G'd's approval through consulting the Urim VeTumim, the breastplate of the High Priest whose letters would flash in answer to questions posed to G'd.