Dogs were used by international forces to deliver vital messages. About a million dogs were killed in action.
Dogs used during WWI included Border Collies, Lurchers, English Sheepdogs, Retrievers and mongrels.
The Airedale Terrier was probably the most common breed used by the British in World War One though.
The Red Cross also commonly used them to find wounded soldiers.
Sergeant Stubby, a Bull Terrier or Boston Terrier, has been called the most decorated war dog of World War I, and the only dog to be nominated for rank and then promoted to sergeant through combat.
Recognized in connection with an exhibition at the Smithsonian Institution.
Among many other exploits, he's said to have captured a German spy. He also became mascot at Georgetown University. Rags was another notable World War I dog.