Warfare is probably the #1 theme here. Ultimately, "The Charge of the Light Brigade" is a poem about a battle. It spends a lot of time describing the confusion, the terror, the bloodshed, and, yes, also the heroism and excitement of armed combat. Notice that most of the images and descriptions in the poem relate to warfare: cannon, bullets, smoke, sabres, etc.
Even though the Russians were half of the battle, the speaker couldn't care less about them. He carefully limits his descriptions of the enemy to keep us completely sympathetic with the Light Brigade.
The relentless repetition of the details of the battlefield is meant to immerse us in the world of the poem, to give us a gut-level sense of the intensity of war.