There was an old saying that commanding was about attacking the opponent by surprise.
Jiang Yi had his dantian sealed up since he was young. He was pathetically weak and often bullied by Jiang Ruhu and his group, but he never gave in. Thus, he had studied many military books, in an attempt to outwit Jiang Ruhu and his gang and beat them up.
Jiang Yunhai had a lot of books in the house as the Grand Elder of the Jiang Clan. Apart from cultivation, Jiang Yi spent most of his time reading so he knew a great deal about the art of war.
The art of warfare didn't have to be the art of war between two troops. Actually, it could be applied in any fight. In addition, the most crucial point of the art of warfare was—craftiness. In other words, attack the opponent by surprise.