Days after Hitler’s suicide a group of American soldiers, French prisoners, and, yes, German soldiers defended an Austrian castle against an SS division—the only time Germans and Allies fought together in World War II. Andrew Roberts on a story so wild that it has to be made into a movie. The most extraordinary things about Stephen Harding’s The Last Battle, a truly incredible tale of World War II, are that it hasn’t been told before in English, and that it hasn’t already been made into a blockbuster Hollywood movie. Here are the basic facts: on 5 May 1945—five days after...