First, a battle is conflict. You need to give the reader a sense of the stakes involved, and what could be lost. What's being fought over, both on a macro level and on a micro level. On the macro level, battles are fought by armies with motives determined by politicians or kings. At the micro level, war is fought by men with motives of their own, from officers to foot soldiers. The biggest mistake would be to stay at the macro level. The reader should be drawn into the story, and to do that, you need them to care about your characters and feel close to them. Write so the reader feels close to the action and the character(s) so they have a rooting interest. Staying at the macro level keeps the reader at a distance.

Second, make it real. Describe on a macro level where they are, how they're armed, and if they have a battle plan. Weapons and the lay of the battlefield will determine how the action scenes play out. Work down toward the micro level, do the men respect their officers, do they have faith in the battle plan, or do they see flaws and feel helpless. Are the men starving and cold, describe the preperations, describe the fear. Remember that the only man in battle who isn't afraid, is one who knows he won't die (those are usually high ranking officers at the back away from action). Show how the fear manifests itself in the few characters you'll follow through the battle.

Third, show the aftermath. The aftermath of a battle can be as hard on the victors as the vanquished, and as difficult as the battle itself; there are wounds to tend to, lost friends, conflicts of conscience over killing, etc. Always remember, it's not about the battle, it's about the men, your characters, and what happens to them both physically and mentally.

Good luck -Craven