As nature gradually swallows up the crumbling concrete walls of a little-known Nazi concentration camp built on a British island, researchers are digging through records to examine how many people died there during the nearly five years of German occupation. The Channel Islands, which lie just off the coast of France, became possessions of the English crown around a thousand years ago. When Germany invaded France in 1940, the British government calculated that the Channel Islands had no strategic value and gave them up without a fight. German troops set up two concentration camps, as well as labor camps, on...