It’s possible to go through an entire writing career without having to send your characters off to war. But even in the most unlikely of genres – “sex and shopping” summer beach novels, or literary novels set in darkest Academe – an unexpected plot turn can have your characters heading for the sound of the guns (or the clash of swords, depending upon the era and the tech level), and next thing you know, there you are, smack dab in the middle of a pitched battle. You may even, if you’re working in some of the more speculative genres, have to make up a battle from scratch.
How, then, to make the ensuing military engagement, if not realistic, at least credible? There are a couple of reliable ways.
One way, of course, is personal experience. If you have it, you know already that you do, and you might as well get all the use out of it that you can. About the only thing you need to remember is that fiction has to be believable, whereas reality is under no such constraint.
The other way, of course, is research. There’s research done the slow way, when you read political history and military history and Sun Tzu and Clausewitz and the memoirs of a lot of people who got out of their various wars alive and wrote about them afterward, and play a lot of war games and maybe do some historical re-creation on the side, and then put all of that together to synthesize your battle.
Then there’s research done the fast way, where you steal a battle outright. This is especially useful when the military aspect of the plot isn’t the main thing – maybe you’re more interested in the romance, or the politics, or the class/race/gender/whatever issues – but you’ve nevertheless found yourself in a corner of the story where the only way out is through this enormous set-piece battle that you somehow have to write.
What you do, at that point, is pick a historical battle from roughly the same era-and-tech-equivalent as your fictional one, and shamelessly use the terrain and maneuvers and eventual outcome of that battle as the template for your own. It helps to pick a relatively obscure engagement – more people than you suspect are likely to recognize the double-envelopment of Cannae, or the defense of Little Round Top at Gettysburg – and to pick a single point of view character and stick as much as possible to just what he or she is experiencing.
And then you don’t tell anybody what battle you stole. If some fan writes you a letter, or corners you at a convention or a book signing, and says, “Hey – wasn’t that space battle in Book Two of your trilogy a rip-off of the Battle of the River Plate, only in space?”, you can give them a big smile and say, “Why, yes, indeed! How clever of you to notice!”