From Wikipedia:
Stubby was found on the Yale campus in 1917 by John Robert Conroy. He was of unknown breed; some sources speculated that he was part Boston Terrier and part Pit Bull, while other sources state that he was in fact a pure bred Olde Boston Bulldog, and his obituary described him as a "Bull dog" (which was at the time synonymous with "American Bull Terrier" and "Pit Bull terrier").
Stubby marched with Conroy and even learned an approximate salute. When Conroy's unit shipped out to France, Stubby was smuggled aboard the transport SS Minnesota.
Stubby served with the 102nd Infantry, 26th (Yankee) Division in the trenches in France for 18 months and participated in four offensives and 17 battles. He entered combat on February 5, 1918 at Chemin des Dames, north of Soissons, and was under constant fire, day and night for over a month.
In April 1918, during a raid to take Seicheprey (Meurthe-et-Moselle), Stubby was wounded in the foreleg by the retreating Germans throwing hand grenades. He was sent to the rear for convalescence, and as he had done on the front was able to improve morale.
When he recovered from his wounds, Stubby returned to the trenches. After being gassed himself, Stubby learned to warn his unit of poison gas attacks, located wounded soldiers in no man's land, and — since he could hear the whine of incoming artillery shells before humans could — became very adept at letting his unit know when to duck for cover.
He was solely responsible for capturing a German spy in the Argonne. Following the retaking of Château-Thierry by the US, the thankful women of the town made Stubby a chamois coat on which were pinned his many medals. There is also a legend that while in Paris with Corporal Conroy, Stubby saved a young girl from being hit by a car. At the end of the war, Conroy smuggled Stubby home.