One of the longest and most dramatic trade routes of the ancient world, the Tea Horse Road carried a crucial exchange for 15 centuries between China and Tibet. China needed war horses tp protect its northern frontier, and Tibet could supply them. When the Tibetans discovered tea in the 7th century, it became a staple of their diet, but its origins are in southeast China, and they had to trade for it. The result was a network of trails covering more than 3,000 kilometres through forest, gorges and high passes onto the Himalayan plateaus, traversed by horse, mule and yak caravans, and human porters. It linked cultures, economies and political ambitions, and lasted until the middle of the 20th century.