Sad Johnlock one-shot. Taking place post- the Sign of Three. How does Sherlock handle the wedding?

John H. Watson is living in London in 1939 when Britain declares war on Germany, and World War Two begins. He enlists on the Allied side as an army doctor, seeking adventure and honor. But in a losing battle, John becomes a prisoner of war in a German camp. Eventually, his path will cross with a wanted criminal with a terrible history. T for language, violence/gore/warfare, etc.