The Molossus dog of the Molossia region of Epirus was the strongest known to the Romans, and was specifically trained for battle. 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.
This site contains all information about Military Working Dogs Breeds.