Overview
Dogs in warfare have a very long history starting in ancient times. From being trained in combat, to their use as the scouts, sentries, messengers, mercy dogs, and trackers, their uses have been varied and some continue to exist in modern military usage.
History
War dogs were used by the Egyptians, Greeks, Persians, Sarmatians, Baganda, Alans, Slavs, Britons, and Romans.
Among the Greeks and Romans, dogs served most often as sentries or patrols, though they were sometimes taken into battle. The earliest use of war dogs in a battle recorded in classical sources was by Alyattes of Lydia against the Cimmerians around 600 BC. The Lydian dogs killed some invaders and routed others. At the Battle of Pelusium (525 BC), Cambyses II deployed dogs, cats, and other animals held sacred by the Egyptians. By putting these animals on the front lines, he was supposedly able to get the Egyptians to cease using their projectile weaponry.
From Wikipedia (CC BY-SA 4.0).