4 ideas
During an attack, Paul Bäumer, trapped in a shell-hole, stabs a French soldier, Gérard Duval, who takes hours to die beside him. Afterward Paul reads the man's papers, sees photographs of his wife and child, learns he was a printer, and promises the corpse to support his family. Back with his comrades the promise dissolves, and he concludes that recognizing the enemy as a man like himself changes nothing about what the front makes him do.
The schoolmaster Kantorek shames his entire class into volunteering with patriotic speeches, and the first day under fire shows the boys that the older generation's words about duty and the Fatherland do not survive contact with reality. Because the adults who sent them were wrong about the one thing they claimed to know, the young soldiers stop trusting any authority built on words. Institutions that spend the young on abstractions lose their right to be believed by those they spend.
Older soldiers have jobs, wives, and settled lives to return to, so the war interrupts them. The eighteen-year-olds had formed nothing before enlisting, so the war becomes the only foundation they have and it destroys them even if they survive. Paul's word for this is 'burnt out': unable to go back, with no future beyond the front.
On leave Paul finds his mother's love, his old room, and his books unchanged, but he can no longer feel his way back into them. Men at the café tell him how the war should be won, and he cannot describe the front to his dying mother either. Read this way, trauma is not only damage inside the soldier. It is also the lack of any shared language between the one who went and the ones who stayed, which leaves the returning veteran estranged at home.