Dogs were used by the ancient Greeks for war purposes, and they were undoubtedly used much earlier in history. During their conquest of Latin America, Spanish conquistadors used Mastiffs to kill warriors in the Caribbean, Mexico and Peru. Mastiffs, as well as Great Danes, were used in England during the Middle Ages, where their large size was used to scare horses to throw off their riders or to pounce on knights on horseback, disabling them until their master delivered the final blow. More recently, canines with explosives strapped to their backs saw use during World War II in the Soviet Army as anti-tank weapons. In other armies, they were used for detecting mines. They were trained to spot trip wires, as well as mines and other booby traps. They were also employed for sentry duty, and to spot snipers or hidden enemy forces. Some dogs also saw use as messengers. A dog employed by the Sanitary Corps during World War I to locate wounded soldiers is shown here. It is fitted with a gas mask.