Dogs in warfare have a long history starting in ancient times. From war dogs trained in combat to their use as scouts, sentries and trackers, their uses have been varied and continue to exist in modern military usage. During WWI dogs were used by international forces to deliver vital messages. Thousands of dogs died in action. Dogs were often used to carry messages in battle. They would be turned loose to move silently to a second handler. This required a dog that was very loyal to two masters, otherwise the dog would not deliver the message on time, or at all. Some messenger dogs also performed other communication jobs, such as pulling telephone lines from one location to another. Search and rescue dogs were used to locate wounded soldiers as early as 1899, their use was not formally documented until WWI, when they served as military medic dogs on the battlefields to locate the wounded and deliver first aid supplies.