The unexplained glory flies above them. (9)
"Glory" here refers to a flag. It is unexplained in this poem because it makes no sense to fight for something as flimsy as a flapping piece of fabric. Rather, it makes no sense to kill people because they happen to be from a different country and fight under a different flag.
Great is the battle-god, great, and his kingdomA field where a thousand corpses lie. (10-11)
The speaker seems patriotic, but only when it comes to the battle-god ("great […] great"). He speaks highly of this god of death, and seems only concerned with satisfying him, rather than worrying about all those dead bodies.
Swift blazing flag of the regiment,Eagle with crest of red and gold (17-18)
Red and gold on a flag? The red makes us think of all the blood on the field of battle, and the gold reminds us of that yellow trench from earlier. A flag equals patriotism, red and gold equal death, therefore patriotism equals death. Okay, so that's maybe a bit of a stretch. But we think it's there in the poem. What do you think?