The motif of diversity and poverty
One of the major issues in this novel is that the lower ranking members of the army are politically and ethnically diverse, but the leadership is aristocratic. The lower members of society are made to do work that the rich soldiers are not subjected to, either because of the political climate back home, or because of some human propensity to favor the rich. The poor, though, are intense, vivid characters who are gritty and competent. Their ethnic and socioeconomic diversity is actually an indication that the poor are unanimously mistreated in Europe.
The War as a symbol
War is a symbol in this novel, because it represents something to the whole cast of characters. War is a sign of horror, because it represents the onslaught of chaos and fear. When they're all just digging ditches, the money obviously comes between them, because the poor realize they're made to do all the real dirty work. But when the war comes, they are all made equal on the battlefield, because the horror of WWI is truly unimaginable to these men, for whom war represented something very different. None of them expected tanks, machine guns, or mustard gas, so the war equalizes all of them.
The weather as a sign
The weather throughout the novel is a portent for war. They are often cold, especially at night, and when the poor are made to dig ditches, they work hard through the cold rain. This is a portent, because the wind and rain foreshadow the coming battle. This means that the men who stand out in the cold rain and work, perhaps they are less astonished when the battle begins—they have not been spoiled by privilege, so they know what to expect from war. The men who party under tends while other men dig ditches, they are understandably very unnerved by the actual battle.
The motif of new weaponry
The basic motif in the book is that over and over again, the axis forces use weapons against the allies that boggle their minds. When mustard gas is tossed into their trenches, none of them know quite how to respond, because it is the first introduction of such chemical warfare to Europe in such an obviously mass produced way. Also, the use of tanks was especially unnerving, because they represented the vulnerability of foot soldiers. What can a foot soldier do to save themselves from an approaching tank? The new weapons add an element of unfathomable horror, because again and again, the battle gets worse than they ever anticipated.
The trip to a village
Potterlo wants to visit his home town village which is in France not that far from the trenches. It's not unimaginable that he would be able to go, by the way—the army often lets its soldiers travel while they are abroad, so when he never gets to go, that's a symbol for a path of life taken away from him by war. He can no longer go back to the way things were, it seems, nor is he allowed to rally himself in his community. He must fight alone, condemned to the battlefield.
