Drugged into Complacency

The opium trade allowed the British many advantageous concessions that would not have been possible in any other case. Without opium, the British would have been far out-traded in any exchange with the Chinese because, in general, the latter had more useful goods to sell, such as silk and tea. Though even these were luxury goods, they were not quite so frivolous or nefarious as opium. At least with cloth, one could do something productive, but opium actually prevented good progress. Even more, if the British did not have access to opium, the goods that they traded would have been far outweighed by what the Chinese had to offer, leaving them with the shorter end of the stick. As it was, the British earned nearly 9,000,000 more in Spanish dollars than the Chinese did from their exchange.
This was a notable disadvantage to China in more ways than one. Now, with this difference in earnings, China had suddenly become dependant on Europe, and Britain in particular, where it had never been so before. Not having experienced this, it did not know to be suspicious. Beyond even this, the trade of Opium was illegal which meant that both the British and the Chinese themselves were ignoring the law. This was empowerment to the British because they now felt that there were no repercussions for such actions, and for the Chinese this could only be a source of weakness and corruption. More and more, the usefullness of the country would falter in the face of addiction. This left them open for easy attack and any argument against growing European power would only provide an excuse for invasion.
This kind of invasion and destruction through trade reminds me a little of trade with the African tribes for slaves in that what passed was a corruption of the pride of each country that weakened them in future. Corruption was present in both, on the part of the country that ultimately fell. Having corruption be the sources of weakness and downfall also hearkens back to the history of China itself - the Ming dynasty fell to the Qing because of revolts against corrupt government. This method may not have been used before, but there were certainly other cases of the country burning from the inside out.