War Dogs have had a role in warfare since their domestication. For the purpose of this entry, we assign the war dog to the Roman Republic (240 B.C.) though Asia and even North America domesticated the dog as early as 10,000 B.C.
The concept of a war dog is an animal that is specifically trained for war to harass and attack the enemy. The war dog was usually highly trained for a period of years by competent trainers whose sole purpose was to breed these lethal animals, sometimes endangering themselves in the process. War dogs were led onto the battlefield by specialty handlers and the handlers would release the dogs on command.
Naturally, columns and formations of troops being attacked or harassed by a platoon of vicious attack dogs would more than likely break formation to repel them, softening the formation for the initial strike by infantrymen.
War dogs continue in some form or another today (nominally as police dogs for the most part - trained to assail but not to kill) and have been used by the ancient Roman Republic, the English against the Celts, by the Spanish in the conquests of the New World, in World War Two by the Soviet Union as 'anti-tank dogs' (explosives-laden dogs trained to run underneath a tank and detonate) and even as recent as the Vietnam War.