Dogs were used as messengers in WW1, carrying orders to the front lines in capsules attached to their bodies.
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.
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.
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."