Ah sorry, I guess yet another case of English people going like "let's take a latin word and change its meaning LOL" :)
> Literally meaning "subordinate", subaltern is used to describe commissioned officers below the rank of captain and generally comprises the various grades of lieutenant.[2]
Commissioned officers are still subordinate to those above them. They just outrank NCOs and warrant officers. In theory, the freshest young ROTC grad can order the Command Sergeant Major of the Army (highest NCO in US Army) to do anything that is legal. In practice, nobody is that dumb. Intelligent lieutenants will give the sergeants under their command orders like "Sergeant, I need you and your troops to take out that enemy emplacement". Minor operational details are left up to the NCOs (who generally have much more combat experience) unless there is a specific reason that the lieutenant needs them to do or avoid some action, and generally those orders would just be the lieutenant following his or her own orders.