A good thesis to write for Ernest Hemingway's "A Farewell to Arms" is that genuine love can only be temporary in a world torn apart by war. This thesis relates to a major theme in the novel, which is an examination of the relationship between pain and love.
The novel uses war as the setting for the story. The main characters, Catherine and Henry, use each other as a distraction from the horrors of war. Henry seeks to run away from the war. Catherine wants to distract herself from the memory of her dead fiancé. Their plan to build a life together distracts them from the pain of war.