Ninjutsu is a Japanese martial art that emerged in the 6th century to train spies (ninjas) during wartime. It originated from techniques taught by Tibetan monks who took refuge in Japan after fleeing persecution in China. The monks lived in the mountain regions of Iga and Koga and taught locals like ronin warriors. Over time, ninjas began working as mercenaries for the Japanese government as spies, continuing until World War 2. In the 1970s, ninjutsu was introduced to the West after practitioners hired a Japanese master to teach them. Today it is practiced for self-defense and is used by some military for its combat techniques.