In the great and complicated equation of modern Asian history, just how important was the variable of opium? This book argues, and in large measure demonstrates, that for China opium was a very large variable indeed. Opium lubricated Western penetration into Asia, especially for the British, and helped significantly to pay for the upkeep and administration of Western colonies there. It was the casus belli that opened imperial China's treaty ports and became the metaphor for the falling dynasty's weakness and disgrace. It was the cash crop that lifted millions of Chinese peasants to new prosperity, and that exposed them to heightened levels of scrutiny and harassment by the state. It was the revenue source indispensable to any number of China's secret societies, warlords, political movements, occupying armies, and “national” governments; as such, it was integral to the power struggles among them. By the early twentieth century, few significant elements of Chinese life were untouched by opium in one way or another.