Tolkien grew up and joined the army when the First World War started in 1914.
But, like many soldiers, he suffered badly from what was known as "shell shock".
This means he was affected by the explosions physically - the thousands of loud bangs from bombs hurt his brain.
But it also hurt him mentally and was upset about what he saw during the fighting. His best friend died in the war.
He was let out of the army becaue he was so sick.
He got a job as lecturer in English Language at the University of Leeds then professor of Anglo-Saxon at Oxford University.