@Kaarme even in this chapter Aug acknowledged that this fight open his eyes, showing that he's become arrogant....
not all kings expert on warfare tho, thats why Minister of Defense is exist, to giving Kings a opinion and suggestion to the kings....
and from previous chapters, Aug is more to the politics rather than warfare...
the thing is, its not them who wage the war... but the higher ups, but they push everything on those kids, since they think "oh those kids already OP, so let them command an army"...
so its not their fault that they suck on it, since it was their first time 'on war' moreover commanding an army...
lol not everyone on earth is expert on military or warfare just by watching tv or movies...
i like watch race, then suddenly i become expert racer? no...
there's no indication that Shin like that kind of stuff... he probably watching movie, cartoon or anime not for "warfare stuff" but watching for the fun... like other peoples
and moreover if he's from Japan, he's more focusing on his live, his works, unless he's "otaku" military
when the soldiers make a mistakes, are the commander free from punishment? no...
the armies under them didn't put the blame on Ultimate magician, but the public will... there's always some provocators or people who didn't like them to stirr the trouble for them from this accident... (that if Author decides to makes the story become 'politics' war)