When the war began, Europe's armies had an understanding of warfare that put the use of cavalry in high regard. Soon, however, the deadly terrain that evolved around trench warfare rendered cavalry attacks nearly useless on the Western Front. But the need for constant resupply, movement of new heavy weaponry, and the transport of troops demanded horse power on a massive scale -- automobiles, tractors, and trucks were relatively new inventions and somewhat rare. British and French forces imported horses from colonies and allies around the world, a near-constant flow of hundreds of thousands of animals across the oceans, headed for war. One estimate places the number of horses killed during the four years of warfare at nearly 8 million. Other animals proved their usefulness as well: Dogs became messengers, sentries, rescuers, and small beasts of burden. Pigeons acted as messenger carriers, and even (experimentally) as aerial reconnaissance platforms. Mules and camels were drafted into use in various war theatres, and many soldiers brought along mascots to help boost morale. Only a couple of decades later, at the onset of World War II, most military tasks assigned to animals were done by machines, and warfare would never again rely so heavily on animal power. I've gathered photographs of the Great War from dozens of collections, some digitized for the first time, to try to tell the story of the conflict, those caught up in it, and how much it affected the world. This entry is part 4 of a 10-part series on World War I.