Sure, "Arms and the Boy" doesn't describe any actual fighting, but it does give us a vivid picture of a young soldier who, apparently, is learning the ropes of warfare. He's young, doesn't know how to use his gun, and seems like an unwitting pawn in some cruel game. In other words, he's in way over his head.
This is an anti-war poem, through and through.
This poem only dislikes wars that have no just cause, wars where it feels like the weapons are the ones doing all the decision-making.