From an article by the BBC news - http://www.bbc.com/news/magazine-24478532
In 1939 in Britain, due to the fear of lack of food for the general populous the National Air Raid Precautions Animals Committee (NARPAC) was formed. It drafted a notice - Advice to Animal Owners. This advise said "If at all possible, send or take your household animals into the country in advance of an emergency." It concluded: "If you cannot place them in the care of neighbours, it really is kindest to have them destroyed."
In one week, 750,000 pets were put down. The first bombing of London in September 1940 prompted more pet owners to rush to have their pets destroyed. Little memorial notices begin appearing in newspapers; "Happy memories of Iola, sweet faithful friend, given sleep September 4th 1939, to be saved suffering during the war. A short but happy life - 2 years, 12 weeks. Forgive us little pal,"