Hoarse, booming drums of the regiment, (6)
War sure is noisy, ain't it? The "booming" of the drums makes us think of bombs, shells, guns, explosions, and other destructive weapons. The speaker is so obsessed with war, or has spent so much time fighting, that even something as simple as a drum resembles a weapon.
Little souls who thirst for fight,These men were born to drill and die (7-8)
War has made the speaker into a very callous man. He doesn't even see his soldiers as real people, with hopes and dreams of their own; they're just "little souls" who were always destined to die on the battlefield. These lines suggest a certain cynicism on the part of our speaker, but they raise a fair point—those who participate in war, the soldiers on the ground, often have little control over their fates. They're at the mercy of far greater forces.
Great is the battle-god, great, and his kingdom—A field where a thousand corpses lie (10-11)
Well this is a scathing critique of war mongering if we ever saw one. The battle-god might as well be a coded reference to anybody who controls, directs, or instigates wars. He's got a fine kingdom all right, a field with a bunch of corpses. Some kingdom. Sheesh.