Privilege
The novel indicates almost immediately that, secretly, the reason that people are made to dig trenches in the rain is because they are poor. Those from more prestigious backgrounds don't get assigned to this kind of dirty work, even in the army, it seems. This is a picture of privilege and the lack thereof. The narrator is united to them by the fact that, back home, they're good old boys, farmers and manual laborers and carpenters and such.
Warfare and the brokenness of human nature
These men are drafted to fight in the Great War, from all over the world, basically, to do warfare. But why are they at war? Certainly there are economic issues. There are political disagreements between powerful nations, sure. Then an assassination. But consider this: Aren't there sometimes when those same factors are there, without war erupting? The novel is a picture of this basic dilemma, because the soldiers are asked to give their human lives for their nations, but it was other people who decided that war was the best option.
Order and chaos
In the army, the characters work more efficiently when they abide by order, because there are less disagreements and more cooperation. The reader can see, therefore, which chaos among the ranks would be disastrous. Warfare is hard work, it turns out, not just manually, but also mentally. Not only are these men being asked to work through their death anxiety to get a job done, the job is extraordinarily hard, and every small distraction could potentially cost them their lives. So they are hyper-aware of where there is order, and where there is chaos, and that comes through to us as a thematic balance.
