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. On this 100-year anniversary, 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. Today's entry is part 4 of a 10-part series on World War I, which will be posted every Sunday until June 29. Come back next week for Part 5.

A single soldier on his horse, during a cavalry patrol in World War I. At the start of the war every major army had a substantial cavalry, and they performed well at first. However, the development of barbed wire, machine guns and trench warfare soon made attacks from horseback far more costly and ineffective on the Western Front. Cavalry units did prove useful throughout the war in other theatres though, including the Eastern Front, and the Middle East. (National Library of Scotland)
Gas attack on the West Front, near St. Quentin 1918 -- a German messenger dog loosed by his handler. Dogs were used throughout the war as sentries, scouts, rescuers, messengers, and more. (Brett Butterworth) #
German soldiers pose near a horse mounted with a purpose-built frame, used to accommodate a captured Russian Maxim M1910 machine gun complete with its wheeled mount and ammunition box. (Brett Butterworth) #
Bandages retrieved from the kit of a British Dog, ca. 1915. (Library of Congress) #
A pigeon with a small camera attached. The trained birds were used experimentally by German citizen Julius Neubronner, before and during the war years, capturing aerial images when a timer mechanism clicked the shutter. (Deutsches Bundesarchiv) #
Unloading a mule in Alexandria, Egypt, in 1915. The escalating warfare drove Britain and France to import horses and mules from overseas by the hundreds of thousands. Vulnerable transport ships were frequent targets of the German Navy, sending thousands of animals to the bottom of the sea. (Bibliotheque nationale de France) #
Sergeant Stubby was the most decorated war dog of World War I and the only dog to be promoted to sergeant through combat. The Boston Bull Terrier started out as the mascot of the 102nd Infantry, 26th Yankee Division, and ended up becoming a full-fledged combat dog. Brought up to the front lines, he was injured in a gas attack early on, which gave him a sensitivity to gas that later allowed him to warn his soldiers of incoming gas attacks by running and barking. He helped find wounded soldiers, even captured a German spy who was trying to map allied trenches. Stubby was the first dog ever given rank in the United States Armed Forces, and was highly decorated for his participation in seventeen engagements, and being wounded twice. (Wikimedia Commons) #
Members of the Royal Scots Greys cavalry regiment rest their horses by the side of the road, in France. (National Library of Scotland) #
At Kemmel, West Flanders, Belgium. The effect of enemy artillery fire upon German ambulances, in May of 1918. (National Archive/Official German Photograph of WWI) #
Red Crescent Hospital at Hafir Aujah, 1916. (Library of Congress) #
A corporal, probably on the staff of the 2nd Australian general hospital, holds a koala, a pet or mascot in Cairo, in 1915. (Australian War Memorial) #
Turkish cavalry exercises on the Saloniki front, Turkey, March of 1917. (National Archives) #
A messenger dog with a spool attached to a harness for laying out new electric line in September of 1917. (National Archive/Official German Photograph of WWI) #
An Indian elephant, from the Hamburg Zoo, used by Germans in Valenciennes, France to help move tree trunks in 1915. As the war dragged on, beasts of burden became scarce in Germany, and some circus and zoo animals were requisitioned for army use. (Nationaal Archief) #
German officers in an automobile on the road with a convoy of wagons; soldiers walk along side the road. (Library of Congress) #
"These homing pigeons are doing much to save the lives of our boys in France. They act as efficient messengers and dispatch bearers not only from division to division and from the trenches to the rear but also are used by our aviators to report back the results of their observation." (WWI Signal Corps Photograph Collection) #
Belgian Army pigeons. Homing pigeon stations were set up behind the front lines, the pigeons themselves sent forward, to return later with messages tied to their legs. (Library of Congress) #
Two soldiers with motorbikes, each with a wicker basket strapped to his back. A third man is putting a pigeon in one of the baskets. In the background there are two mobile pigeon lofts and a number of tents. The soldier in the middle has the grenade badge of the Royal Engineers over the chevrons which show he is a sergeant. (National Library of Scotland) #
A message is attached to a carrier pigeon by British troops on the Western Front, 1917. One of France's homing pigeons, named Cher Ami, was awarded the French "Croix de Guerre with Palm" for heroic service delivering 12 important messages during the Battle of Verdun. (Bibliotheque nationale de France) #
A draft horse hitched to a post, its partner just killed by shrapnel, 1916. (Bibliotheque nationale de France) #
The feline mascot of the light cruiser HMAS Encounter, peering from the muzzle of a 6-inch gun. (Australian War Memorial) #
General Kamio, Commander-in-Chief of the Japanese Army at the formal entry of Tsing-Tau, December, 1914. The use of horses was vital to armies around the world during World War I. (Paul Thompson/New York Times) #
Belgian refugees leaving Brussels, their belongings in a wagon pulled by a dog, 1914. (Bibliotheque nationale de France) #
Australian Camel Corps going into action at Sharia near Beersheba, in December of 1917. The Colonel and many of these men were killed an hour or so afterward. (Australian official photographs/State Library of New South Wales) #
On the Western Front, a dead German artilleryman and several draft horses, ca. 1918. Exact figures are hard to come by, but an estimated 8 million horses died during the four years of war. (Library of Congress) #
A soldier and his horse in gas masks, ca. 1918. (Woodrow Wilson Presidential Library) #
German Red Cross Dogs head to the front. (Library of Congress) #
An episode in Walachia, Romania. (Der Weltkrieg im Bild/Upper Austrian Federal State Library) #
Belgian chasseurs pass through the town of Daynze, Belgium, on the way from Ghent to meet the German invasion. (Library of Congress/Underwood & Underwood, War of the Nations, New York Times) #
The breakthrough west of St. Quentin, Aisne, France. Artillery drawn by horses advances through captured British positions on March 26, 1918. (National Archive/Official German Photograph of WWI) #
Western Front, shells carried on horseback, 1916. (Bibliotheque nationale de France) #
Camels line a huge watering station, Asluj, Palestinian campaign, 1916. (Library of Congress) #
A British Mark V tank passes by a dead horse in the road in Peronne, France in 1918. (Nationaal Archief) #
A dog-handler reads a message brought by a messenger dog, who had just swum across a canal in France, during World War I. (National Library of Scotland) #
Horses requisitioned for the war effort in Paris, France, ca. 1915. Farmers and families on the home front endured great hardship when their best horses were taken for use in the war. (Library of Congress) #
In Belgium, after the Battle of Haelen, a surviving horse is used in the removal of dead horses killed in the conflict, 1914. (Bibliotheque nationale de France) #
A dog trained to search for wounded soldiers while under fire, 1915. (Bibliotheue nationale de France) #
Algerian cavalry attached to the French Army, escorting a group of German prisoners taken in fighting in the west of Belgium. (Library of Congress/Underwood & Underwood, War of the Nations, New York Times) #
A Russian cossack, in firing position, behind his horse, 1915. (Bibliotheque nationale de France) #
Serbian artillery in action on the Salonika front in December of 1917. (Nationaal Archief) #
A horse strapped and being lowered into position to be operated on for a gunshot wound by 1st LT Burgett. Le Valdahon, Doubs, France. (CC BY Otis Historical Archives) #
6th Australian light-horse regiment, marching in Sheikh Jarrah, on the way to Mount Scopus, Jerusalem, in 1918. (Library of Congress) #
French cavalry horses swim across a river in northern France. (Underwood & Underwood) #
Dead horses and a broken cart on Menin Road, troops in the distance, Ypres sector, Belgium, in 1917. Horses meant power and agility, hauling weaponry, equipment, and personnel, and were targeted by enemy troops to weaken the other side -- or were captured to be put in use by a different army. (National Library of New Zealand) #
War animals carrying war animals -- at a carrier pigeon communication school at Namur, Belgium, a dispatch dog fitted with a pigeon basket for transporting carrier pigeons to the front line. (National Archives/Official German Photograph) #