It is important to analyse this statue as a piece of propaganda for Augustus. It was about projecting his power and might to those around him in a way that would clearly emphasise his own strength and achievements and present him as a force to be reckoned with. There is of course his mien but it is important to identify as well the way in which his military prowess is evidenced.
There are three things in particular that make Augustus seem like a great military leader. First, he is wearing his military garbs. This shows that he is not just a leader, but a military leader. Second, he has his consular baton in his left arm. Finally, he has his right hand up, which was a position of a general addressing his army.
Augustus wears a cuirass, or traditional military breastplate. It is decorated with various scenes of Roman military triumphs, including victories over barbarian peoples. He is also holding a spear, an obvious symbol of military prowess. These are pretty easy to interpret. But other iconography is less obvious. Even in the Republic, Roman claims to power and authority were rooted in their family ancestry. Senators, for example, always came from a handful of aristocratic families. The Cupid riding on the back of the dolphin next to Augustus's leg, for example, connects his geneology back to Venus, a central claim for anyone who would rule Rome.
First of all, just the look of the statue conveys power and confidence. It makes him look commanding and regal. Then you start looking at the details and it's like an ad for Augustus. It has symbols referring to all of his military victories and it has symbols that are meant to show his connections to the gods. So, both in the general look of the statue and in its details, we get a sense of Augustus as a very powerful leader and military man.