Very easy to unlock, even early on. I got it between missions 3 and 4.
Simply recruit enough units to fill all of the available slots on the Third Army Units page. After a mission or two, you will have upwards of 20,000 prestige points so can fill all the slots by recruiting a load of standard infantry units at 1,000 points each.
Crucially, you should then quit straight to dashboard and reload. It will not have saved after you bought the additional units, so you will be back where you were beforehand with prestige points still available.