After the horrors of battle, Paul and the other soldiers become
...insensible dead men who through some trick, some dreadful magic, are still able to run and kill.
As he stands on sentry, the benumbed Paul, like an experienced old man, finds himself besieged by memory, that sensation that controls people rather than the other way around. In his "vast, inapprehensible melancholy," Paul recalls the world that he has lost, a world of youthful ideals that is no longer attainable. He narrates, "We could never regain the old intimacy with those scenes."
Paul recalls the grove of poplar trees under which he and his friends felt safe. The clean, fresh air, "the pure fragrance of the water" and " the melody of the wind" held the boys' fascination. He and his friends dearly loved these two qualities of the poplar grove, so much so that as he stands sentry, Paul feels a pang in his heart for what he has lost in his young heart.