The heroes in Remarque's novel are chased by the ghosts of their past. The shadows of ugly and horrific events of the second World War are not the stalkers you can easily get rid of, even if you ran away from them to the paradise, which the USA were for poor victims of cruel ideology.
The novel tells us how a purpose of life changes for a person depending on the environment, how to LIVE AFTER you were lucky enough to survive, how to live peaceful life and reorientate your goals when the memories are so strong. Many people can't live normal life after significant stressful events: happiness is transient, while fear can stay with you forever. Remarque shows that sometimes the most severe battles are held inside our mind. To live, not exist, man must conquer not only the external conditions, but the forces within himself, forces that pull him down in the past