>>509029724
>They alvo leave one flank open to let enemies retreat under heavy fire.
This strategy dates back to Sun Tzu (perhaps even older). Letting your enemy be fooled into thinking they have a safe exit that is actually under heavy fire is better than completely surrounding them, because in the latter case they will fight like a cornered animal because they'll know they are one. Its better to fool them into thinking they're not against a wall.