An Eagle in the Snow by Michael Morpurgo - old hardcover
1940. Barney and his mother, their home destroyed by bombing, are travelling to the country when their train is forced to shelter in a tunnel from attacking German planes. There, in the darkness, a stranger on the train begins to tell them a story. A story about Bobby Byron, the most decorated soldier of WW1, who once had the chance to end the war before it even began, and how he tried to fix his mistake. But sometimes the right thing is hard to see, and even harder to live with.