Renowned World War I canine hero, Stubby, is photographed on the battlefield wearing a coat, hat and collar, with a gun at his side. Stubby once saved multiple soldiers when he roused them from their sleep after a German mustard gas attack.

The phrase “war dog” is a technical one, and did not apply to U.S. dogs at this time, according to Kathleen Golden, curator of the National Museum of American History’s Division of Armed Forces History. “It wasn’t until World War II that the United States began using dogs officially,” she says. Before then, they were considered “mascots.”

In 1922, a bulldog named Jiggs was inducted into the U.S. Marine Corps by General Smedley Butler. He later was promoted to Sergeant Major Jiggs. Germans called the U.S. Marines “Teufel-Hunden,” or “Devil Dogs,” inspiring Jiggs and a succession of other decorated bulldog mascots.

Belgians decorated their dogs with the hats of German soldiers in 1914, after the dogs were used to move light artillery and machine guns on small carts. Ronald Aiello, president of the United States War Dogs Association, says that German shepherds, bulldogs, Airedale terriers and retrievers were the most commonly used dog breeds during World War I.

Terriers were a preferred breed during the war, Golden says, for their loyalty, rodent-hunting skills and friendly demeanors. New Zealander soldier W. J. Batt poses here with a regimental mascot at Walker’s Ridge during the Gallipoli campaign in Turkey on April 30, 1915.

A German Army dog is photographed wearing a hat and glasses, with a pair of binoculars around his neck. The Germans began using dogs in an official capacity during wartime in the late 19th century, not long before the start of World War I. The Allied Forces had at least 20,000 dogs on the battlefields of World War I, while the Central Powers—primarily Germany—had about 30,000.

Golden says that during World War I, “Dogs were primarily used as messengers.” On July 5, 1916, this messenger dog used by the British Army in Flanders, Belgium runs to the front with urgent messages.

Message dogs were often outfitted with collars that had attached cylinders. Here, a sergeant of the Royal Engineers places a message into the cylinder on August 28, 1918, at Etaples, France.

Messenger dogs such as “Wolf,” an Alsatian, often had to negotiate dangerous obstacles, including barbed wire entanglements. Here, Wolf clears a fence at the Western Front in Flanders, Belgium.

While horses were often used to haul heavy guns and other equipment, teams of dogs would also be recruited for hauling weapons and other objects. Italian soldiers oversee dogs performing such work in 1917.

Dogs, with their keen sense of hearing, endured frequent exposure to gunfire and other loud sounds during World War I. This dog belonged to Captain Richardson of the U.K., who brought his canine companion with him to the trenches in 1914.

Visual cues were critical for dogs on missions during World War I. German soldiers in 1916 appear to point something important out to a dog serving as a messenger in the field.

World War I dogs, especially terriers, proved to be productive rat hunters. That was an invaluable skill in the war’s rat-infested trenches. Here, a terrier poses with some of his kill near the front lines of France in May 1916.

In France in 1915, a dog is dressed up as a German soldier—complete with pipe and goggles—to the amusement of soldiers marching by.

Resting in a wooden building at an airfield, German military pilots smoke pipes and chat alongside their canine companion. Dogs were great “morale boosters” for troops on both sides of the battlefields during World War I, Golden says.

Mascots such as “Doreen,” an Irish wolfhound, were often brought to memorial services. World War I was one of the deadliest conflicts in human history, with military and civilian casualties estimated at over 16 million. Doreen was a mascot of the 1st Battalion of the Irish Guards.

These dogs are armed with first aid equipment and stimulants as they help search for wounded soldiers in no man’s land.

Aiello explains that “dogs were trained to find the wounded or dying soldiers on the battlefield. This would let the medics know who was still alive so the injured could get immediate medical treatment.” This dog finds a wounded soldier lying under a tree in Austria, July 1916.

A French Red Cross dog demonstrates his climbing skills by scaling a 6-foot-high wall. Dogs often had to maneuver over comparable obstacles while searching for wounded soldiers.

“I think that Red Cross dogs were the heroes of World War I,” Aiello says. The dogs would not only locate wounded soldiers, as shown in this 1917 image, they would also help to transport them from the battlefield.

A French sergeant and a dog, both wearing gas masks, marched to the front lines. Many dogs were injured by toxic gas. Still others died from exposure to chemical agents like chlorine and phosgene.

During the spring of 1917 a French messenger dog wearing a gas mask runs through a cloud of poisonous gas.

German soldiers and their dogs wore gas masks as well. The Germans were the first to use such chemical weapons during this war, releasing clouds of poisonous chlorine at Ypres, Belgium in April 1915.

A German Army dog manages to leap over a trench in France while delivering a message from one outpost to another. Thousands of dogs died while serving in World War I, often while delivering messages. Once a message was delivered, the dog would be turned loose to move silently to a second handler.

Two soldiers captured a pair of German dogs during World War I. The canines were named Crown Prince and Kaiser Bill. The men, wounded in battle, posed with the dogs before returning with them to the United States.

This dog, photographed in 1915 in a trench at Flanders, Belgium, and other military dogs have safeguarded and aided people on battlefields from before World War I to the present, says Aiello, who was deployed to Vietnam in 1966 with his own canine companion, Stormy. “They protect our troops and would die for us.”