During Britain's Imperial era, they expanded their trade world-wide and boasted the greatest Navy in the world. One of the goods they were selling was opium, and this drug quickly became popular in Canton and the coast of China despite the opposition of the local and national governments of China. Great Britain was upset by Chinese attempts to disrupt this lucrative trade and declared war to force the market to remain open. The long and the short of it is that Britain won. They forced China to concede the island of Hong Kong and into a 99 year lease on the New Territories (a region geographically much larger than the island.)