Informal empire, like many imperial relationships, is difficult to classify and reduce to a prescriptive definition. In the instance of the British informal empire, the character of the relationship varied widely. The Chinese intensely opposed any foreign control over China or its economy, and both the First and Second Opium Wars were fought between China and several Western powers, resulting in Chinese defeat and the increase of both formal and informal foreign power. South American governments were often willing partners in the extension of British commercial influence, but military action was sometimes taken against those who tried to apply protectionist policies (see for example, the Anglo-French blockade of the River Plate). Informal empire is an important concept required to adequately explain the reach and influence of empire, and in the case of the British Empire, is vital to any holistic account of the British imperial experience and intrinsic to describing the interests and purposes of the Empire as a whole. Informal empire, far from being distinctive and separate from formal empire, is often bound up with formal imperial interests. For example, the British informal empire in China was a product of Company rule in India, as the East India Company used its Indian territories to grow opium, which was then shipped to Chinese ports. By 1850, the Chinese opium trade accounted for up to 20% of the revenue of the entire British Empire, serving as the most profitable single commodity trade of the 19th century. As Timothy Brook and Bob Wakabayashi write of opium trade, “The British Empire could not survive were it deprived of its most important source of capital, the substance that could turn any other commodity into silver.”