During the first four days of World War II, over 400,000 dogs and cats — some 26 percent of London’s pets — were slaughtered, a number six times greater than the number of civilian deaths in the UK from bombing during the entire war. It was a calm and orderly massacre. One animal shelter had a line stretching half a mile long with people waiting to turn their animals over to be euthanized. None of this was done of out any real necessity. A new book by the historian Hilda Kean, sets out to understand how and why these horrific events took place. Reviewed by Colin Dickey.