It's likely you're thinking of
"Aftermath" by Siegfried Sassoon. This poem was written in 1919, just after the end of World War I, and is one of his most famous and impactful works. It captures the disillusionment and despair felt by many soldiers returning from the war, struggling to readjust to a life that felt drastically different from the one they knew before.
However, Sassoon wrote many poems after World War I, so if you have another poem in mind, please provide more context or information about it.