The brutality of war
Obviously the most important theme of the novel is its depiction of gore and violence. The novel is a snapshot of an aspect of human nature that doesn't make sense to peaceful people in peaceful times, but those terrifying wars really happened. So the novel merely observes them through Jim's quiet, committed attention to detail.
Existentialism as the product of WWI
Poor Imogen back on the beautiful Gold Coast is left in the wake of Jim's death, and she realizes two things from Jim's legacy: First she learns the difficult lesson of meaninglessness. All of Jim's idyllic participation in life and nature did not immunize him from the gruesome, violent death he suffered. Not only that, but with news of the trench warfare, the entire Western world seems to be falling apart.
However, she also realizes that the legacy of Jim's observation of the natural world is still powerful, even in spite of meaninglessness. In this way, she successful navigates her new existentialism by finding something to provide her with meaning in an essentially meaningless life.
The beauty and power of nature
One of the most ironic aspects of life in the trenches is that for the most part, the rest of the earth's natural processes continue. The earth doesn't stop for human warfare, but instead the crows continue their regular flight patterns, and farmers plant crops in the fresh soil unearthed by bombs.
Jim as a religious martyr
One way to understand the novel is that Jim is actually like a religious martyr. Martyrdom is when a prophetic person experiences the true horror of human evil and dies from it. The idea comes from the Greek word 'marturo' which simply means to witness. One good reason to understand Jim as a martyr is that he spent his normal life paying attention to small things, and appreciating the infinite beauty of nature. That attention makes him especially able to "witness" another thing: human evil and the horror of warfare.
