why do you think he was trained in the art of war

Training in the art of war would have been a tradition.

"Our whole district is a kind of militia: on a certain signal given, such as the firing of a gun at night, they all rise in arms and rush upon their enemy."

"I was trained up from my earliest years in the art of war, my daily exercise was shooting and throwing javelins, and my mother adorned me with emblems after the manner of our greatest warriors."