This is the 10618th most frequent Russian word.
военному
"Военному" translates to "to the military" or "to the serviceman" in English, depending on context.
In this sentence, 'военному' is in the dative case and denotes 'to the military person', serving as an indirect object to the verb 'дал' (gave).
Я дал книгу военному.
I gave the book to the military person.
Here, 'военному' is used as an adjective in dative case modifying 'уставу' (regulations), indicating that the regulations pertain to the military.
По военному уставу нельзя оставлять пост.
According to the military regulations, leaving the post is prohibited.
In this context, 'военному' is part of the adverbial phrase 'по-военному', which signifies an action performed in a manner characteristic of the military.