If you want to write in a genre, you have to read that genre. Experts in the industry are consistent with their advice that you must be an avid reader of a genre to write well in it. In addition to the suggested fiction, I've included a number of books about the military itself from different eras, as that is the kind of knowledge one needs to be successful in this genre.
When you think of history, do you think of battles won and lost? Did you think Gandalf's charge with a thousand Rohirrim heavy horse ill-advised since they faced massed infantry with set spears? Did you love Game of Thrones but choked on your drink when they put the trebuchet in front of their defensive positions?