Editor's note: This is the first of a five-part series.

As long as men have been fighting wars, dogs have likely been somewhere on or near the battlefield. And more often than not, dogs have contributed bravely on the front lines, whether officially trained to do so or motivated by loyalty to soldiers.

The history of war dogs is deep: The Corinthians used them with success against the Greeks. The Romans used dogs to guard their legions and raise alarms, as did Attila the Hun, who placed them around his camps for added protection.

The United States military has lagged behind the rest of the world's armies in using dogs, even though the idea was introduced early on. Benjamin Franklin made a somewhat lackluster attempt to advocate for using dogs (though more as weapons) in 1755.

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Beginning with the Revolutionary War and through World War I, dogs had a mostly unofficial presence alongside American soldiers, coming to combat either as a beloved pet of a general, as a mascot, or as the stray-made-companion of an obliging soldier.

It wasn't until the onset of World War II that the U.S. War Department, emulating successful war dog programs in Europe, finally set into motion the military dog program that would evolve (and lapse and evolve again) over the next several decades. Started in World War II and continuing through Korea and Vietnam, today the Military Working Dog Program deploys dogs to Iraq and Afghanistan.

In the coming days, we take a look back at a handful (of the many thousands) of war dogs whose stories are powerful testaments to the important roles they played in saving lives—and lifting spirits. (Read "The Dogs of War" in the June issue of National Geographic magazine.)

Ernest Harold Baynes, a reporter who documented the use of animals during World War I, wrote, "The fame of the war dogs may well rest on the splendid work they actually did; it needs no support from the stories of what some of the sentimentalists would like to believe they did."

View Images French troops endure shellfire during the 1916 Battle of Verdun. PHOTOGRAPH BY GENERAL PHOTOGRAPHIC AGENCY, GETTY

Satan Saves the Day

During World War I, at the 1916 Battle of Verdun, a small contingent of French soldiers found themselves boxed in by German forces.

They had been told by the French command to hold their position until reinforcements could be sent. For days they had managed to hold off the Germans, but no one had come to relieve them. Telephone and telegraph lines were down, and no homing pigeons remained to send word.

The scorched and cratered terrain beyond their trenches was too exposed for any human to cross—seven men had already been cut down trying to deliver messages to command. And although one dog had managed to successfully deliver seven messages, he too had been killed.

With food and ammunition depleted and the men's hopes waning, the Germans unleashed a fresh onslaught of artillery and gunfire. The French troops cautiously peered over the top of their trenches. A large, black animal was bounding in their direction. From a distance it was difficult to tell exactly what the charging four-legged creature was. It was wearing a monstrous gas mask, and something was stretched across its shoulders that extended almost like wings.

View Images A dog leaps over an Allied trench to deliver a message. Dogs were especially well suited to navigating the obstacles of trench warfare. PHOTOGRAPH BY BAIN NEWS SERVICE/NATIONAL GEOGRAPHIC SOCIETY/CORBIS

Then one of the soldiers, a handler named Duval, recognized the animal as his own—a messenger dog named Satan. Duval called out to the dog, urging him on. Leaping over the cratered earth, Satan raced toward the sound of his handler's voice so fast that some of the men later swore he was flying.

The Germans unloaded their arsenal in an all-out attempt to stop this one dog. But Satan continued on, maneuvering in the crisscross pattern he'd been trained to follow, even as bullets snapped the air around him and exploding shells threw up shrapnel and chunks of smoking earth.

A bullet clipped the dog and he stumbled. Then another caught him in the leg, breaking it, and he faltered again, this time hitting the ground.

Seeing his dog go down, Duval climbed out of the trench, exposing himself to enemy fire so that he could call once more to Satan. Duval was shot dead within seconds. But, having heard his handler's voice again, Satan mustered the strength to lift himself off the ground. He started to run again, this time on three legs, his lame limb hanging useless as he ran until, finally, he reached the safety of the French trenches.

The men lifted the limping dog and gently removed the mask, pulling a tube from around his neck to read the message inside: "For God's sake, hold on. We will send troops to relieve you tomorrow." The winglike contraption on Satan's back was a harness balancing two small baskets over the dog's shoulders, each one containing a carrier pigeon.

The French commander scrawled two identical notes describing the German battalion's position. The notes were put in small metal tubes and tied to the pigeons' legs. The two birds lifted into the air, soaring into the sky. The German snipers were waiting for them. A shot picked off the first bird, but the other somehow made it through the spray, flying in the direction of its coop.

Soon the sound of roaring French guns could be heard. The message had been received.

Coming tomorrow: Caesar in the Pacific